Fixes CVE-2015-8868 (heap overflow).

Upstream source:
https://cgit.freedesktop.org/poppler/poppler/commit/?id=b3425dd3261679958cd56c0f71995c15d2124433

From b3425dd3261679958cd56c0f71995c15d2124433 Mon Sep 17 00:00:00 2001
From: Albert Astals Cid <aacid@kde.org>
Date: Tue, 22 Dec 2015 22:50:33 +0100
Subject: Do not crash on invalid files

Bug #93476

diff --git a/poppler/Function.cc b/poppler/Function.cc
index 67283df..ee5afc1 100644
--- a/poppler/Function.cc
+++ b/poppler/Function.cc
@@ -577,6 +577,10 @@ ExponentialFunction::ExponentialFunction(Object *funcObj, Dict *dict) {
       goto err2;
     }
     n = obj1.arrayGetLength();
+    if (unlikely(n > funcMaxOutputs)) {
+      error(errSyntaxError, -1, "Function's C0 array is wrong length");
+      n = funcMaxOutputs;
+    }
     for (i = 0; i < n; ++i) {
       obj1.arrayGet(i, &obj2);
       if (!obj2.isNum()) {
-- 
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