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='2019-11-08 21:32:45 +0100'>2019-11-08</span></td><td><a href='/guix/commit/gnu/services/audio.scm?id=970cb5ceceaa85765230a9f896a43783cdcb4e6c'>services: mpd: Connect to the user's PulseAudio socket.</a><span class='msg-avail'>...</span></td><td>Robert Smith</td></tr>
<tr><td><span title='2019-10-29 15:14:22 +0100'>2019-10-29</span></td><td><a href='/guix/commit/gnu/services/audio.scm?id=878e0e1bb0d15809c6582436d53498f3e6385524'>gnu: mpd-service-type: Support configuration of outputs.</a><span class='msg-avail'>...</span></td><td>Ricardo Wurmus</td></tr>
<tr><td><span title='2019-03-15 23:27:59 +0100'>2019-03-15</span></td><td><a href='/guix/commit/gnu/services/audio.scm?id=c71b7b5b3740177f0df6d3565dcc356ceda1acba'>services: mpd: add more configuration options</a><span class='msg-avail'>...</span></td><td>nixo</td></tr>
<tr><td><span title='2017-11-05 15:29:52 +0100'>2017-11-05</span></td><td><a href='/guix/commit/gnu/services/audio.scm?id=21b71b01e9c2264c31ef30451c152bb38cc9e635'>services: Add 'description' fields.</a><span class='msg-avail'>...</span></td><td>Ludovic Courtès</td></tr>
<tr><td><span title='2017-08-31 14:43:26 +0200'>2017-08-31</span></td><td><a href='/guix/commit/gnu/services/audio.scm?id=b6dc69af1de806e8b3a70e9a1afe0e1590da5480'>services: mpd: Remove the mpd-file field.</a><span class='msg-avail'>...</span></td><td>Peter Mikkelsen</td></tr>
<tr><td><span title='2017-08-13 07:36:32 +0100'>2017-08-13</span></td><td><a href='/guix/commit/gnu/services/audio.scm?id=06465d2ba4291eb2046c90c3977a295a9b7c434b'>gnu: Add mpd service.</a><span class='msg-avail'>...</span></td><td>Peter Mikkelsen</td></tr>