Fix <https://cve.mitre.org/cgi-bin/cvename.cgi?name=CVE-2019-9169>.

Taken from this upstream commit, sans ChangeLog updates:
https://sourceware.org/git/gitweb.cgi?p=glibc.git;a=commitdiff;h=583dd860d5b833037175247230a328f0050dbfe9

diff --git a/posix/regexec.c b/posix/regexec.c
index 91d5a797b8..084b1222d9 100644
--- a/posix/regexec.c
+++ b/posix/regexec.c
@@ -1293,8 +1293,10 @@ proceed_next_node (const re_match_context_t *mctx, Idx nregs, regmatch_t *regs,
 	      else if (naccepted)
 		{
 		  char *buf = (char *) re_string_get_buffer (&mctx->input);
-		  if (memcmp (buf + regs[subexp_idx].rm_so, buf + *pidx,
-			      naccepted) != 0)
+		  if (mctx->input.valid_len - *pidx < naccepted
+		      || (memcmp (buf + regs[subexp_idx].rm_so, buf + *pidx,
+				  naccepted)
+			  != 0))
 		    return -1;
 		}
 	    }
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<tr><td><span title='2019-05-07 12:18:44 +0200'>2019-05-07</span></td><td><a href='/guix/commit/tests/uuid.scm?id=6c5790a2faf8ffc401e43b13425a707394e40874'>uuid: 'fat-uuid-&gt;string' preserves leading zeros.</a><span class='msg-avail'>...<span class='msg-tooltip'>Fixes &lt;https://bugs.gnu.org/35582&gt;.
Reported by sirgazil &lt;sirgazil@zoho.com&gt;.

Previously, leading zeros would be removed, leading to an "invalid"
UUID:

  (uuid-&gt;string (uuid "00CA-050E" 'fat32))
  ⇒ "CA-50E"
  (string-&gt;uuid "CA-50E" 'fat32)
  ⇒ #f

* gnu/system/uuid.scm (fat-uuid-&gt;string): Pad digits with zeros.
* tests/uuid.scm ("uuid, FAT32, leading zeros preserved"): New test.
</span></span></td><td>Ludovic Courtès</td></tr>
<tr><td><span title='2018-05-19 00:14:52 +0200'>2018-05-19</span></td><td><a href='/guix/commit/tests/uuid.scm?id=263c9941a1e523b360ca9f42d1ed6b11e6e6e285'>uuid: 'uuid' returns #f when 'string-&gt;uuid' returns #f.</a><span class='msg-avail'>...<span class='msg-tooltip'>* gnu/system/uuid.scm (uuid): When STR is not a literal, return #f when
'string-&gt;uuid' returns #f.
* tests/uuid.scm ("uuid, dynamic value"): New test.
</span></span></td><td>Ludovic Courtès</td></tr>