Fix CVE-2020-10531:

https://cve.mitre.org/cgi-bin/cvename.cgi?name=CVE-2020-10531

Patch copied from upstream source repository (changes to the test suite
are commented out):

https://github.com/unicode-org/icu/commit/b7d08bc04a4296982fcef8b6b8a354a9e4e7afca

From b7d08bc04a4296982fcef8b6b8a354a9e4e7afca Mon Sep 17 00:00:00 2001
From: Frank Tang <ftang@chromium.org>
Date: Sat, 1 Feb 2020 02:39:04 +0000
Subject: [PATCH] ICU-20958 Prevent SEGV_MAPERR in append

See #971
---
 icu4c/source/common/unistr.cpp          |  6 ++-
 icu4c/source/test/intltest/ustrtest.cpp | 62 +++++++++++++++++++++++++
 icu4c/source/test/intltest/ustrtest.h   |  1 +
 3 files changed, 68 insertions(+), 1 deletion(-)

diff --git a/icu4c/source/common/unistr.cpp b/icu4c/source/common/unistr.cpp
index 901bb3358ba..077b4d6ef20 100644
--- a/icu4c/source/common/unistr.cpp
+++ b/icu4c/source/common/unistr.cpp
@@ -1563,7 +1563,11 @@ UnicodeString::doAppend(const UChar *srcChars, int32_t srcStart, int32_t srcLeng
   }
 
   int32_t oldLength = length();
-  int32_t newLength = oldLength + srcLength;
+  int32_t newLength;
+  if (uprv_add32_overflow(oldLength, srcLength, &newLength)) {
+    setToBogus();
+    return *this;
+  }
 
   // Check for append onto ourself
   const UChar* oldArray = getArrayStart();
#diff --git a/icu4c/source/test/intltest/ustrtest.cpp b/icu4c/source/test/intltest/ustrtest.cpp
#index b6515ea813c..ad38bdf53a3 100644
#--- a/icu4c/source/test/intltest/ustrtest.cpp
#+++ b/icu4c/source/test/intltest/ustrtest.cpp
#@@ -67,6 +67,7 @@ void UnicodeStringTest::runIndexedTest( int32_t index, UBool exec, const char* &
#     TESTCASE_AUTO(TestWCharPointers);
#     TESTCASE_AUTO(TestNullPointers);
#     TESTCASE_AUTO(TestUnicodeStringInsertAppendToSelf);
#+    TESTCASE_AUTO(TestLargeAppend);
#     TESTCASE_AUTO_END;
# }
# 
#@@ -2310,3 +2311,64 @@ void UnicodeStringTest::TestUnicodeStringInsertAppendToSelf() {
#     str.insert(2, sub);
#     assertEquals("", u"abbcdcde", str);
# }
#+
#+void UnicodeStringTest::TestLargeAppend() {
#+    if(quick) return;
#+
#+    IcuTestErrorCode status(*this, "TestLargeAppend");
#+    // Make a large UnicodeString
#+    int32_t len = 0xAFFFFFF;
#+    UnicodeString str;
#+    char16_t *buf = str.getBuffer(len);
#+    // A fast way to set buffer to valid Unicode.
#+    // 4E4E is a valid unicode character
#+    uprv_memset(buf, 0x4e, len * 2);
#+    str.releaseBuffer(len);
#+    UnicodeString dest;
#+    // Append it 16 times
#+    // 0xAFFFFFF times 16 is 0xA4FFFFF1,
#+    // which is greater than INT32_MAX, which is 0x7FFFFFFF.
#+    int64_t total = 0;
#+    for (int32_t i = 0; i < 16; i++) {
#+        dest.append(str);
#+        total += len;
#+        if (total <= INT32_MAX) {
#+            assertFalse("dest is not bogus", dest.isBogus());
#+        } else {
#+            assertTrue("dest should be bogus", dest.isBogus());
#+        }
#+    }
#+    dest.remove();
#+    total = 0;
#+    for (int32_t i = 0; i < 16; i++) {
#+        dest.append(str);
#+        total += len;
#+        if (total + len <= INT32_MAX) {
#+            assertFalse("dest is not bogus", dest.isBogus());
#+        } else if (total <= INT32_MAX) {
#+            // Check that a string of exactly the maximum size works
#+            UnicodeString str2;
#+            int32_t remain = INT32_MAX - total;
#+            char16_t *buf2 = str2.getBuffer(remain);
#+            if (buf2 == nullptr) {
#+                // if somehow memory allocation fail, return the test
#+                return;
#+            }
#+            uprv_memset(buf2, 0x4e, remain * 2);
#+            str2.releaseBuffer(remain);
#+            dest.append(str2);
#+            total += remain;
#+            assertEquals("When a string of exactly the maximum size works", (int64_t)INT32_MAX, total);
#+            assertEquals("When a string of exactly the maximum size works", INT32_MAX, dest.length());
#+            assertFalse("dest is not bogus", dest.isBogus());
#+
#+            // Check that a string size+1 goes bogus
#+            str2.truncate(1);
#+            dest.append(str2);
#+            total++;
#+            assertTrue("dest should be bogus", dest.isBogus());
#+        } else {
#+            assertTrue("dest should be bogus", dest.isBogus());
#+        }
#+    }
#+}
#diff --git a/icu4c/source/test/intltest/ustrtest.h b/icu4c/source/test/intltest/ustrtest.h
#index 218befdcc68..4a356a92c7a 100644
#--- a/icu4c/source/test/intltest/ustrtest.h
#+++ b/icu4c/source/test/intltest/ustrtest.h
#@@ -97,6 +97,7 @@ class UnicodeStringTest: public IntlTest {
#     void TestWCharPointers();
#     void TestNullPointers();
#     void TestUnicodeStringInsertAppendToSelf();
#+    void TestLargeAppend();
# };
# 
# #endif