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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<div class='path'>path: <a href='/guix/log/?id=60562ec26bb512dbf9ea44de0df8b48f1f6e1880'>root</a>/<a href='/guix/log/gnu?id=60562ec26bb512dbf9ea44de0df8b48f1f6e1880'>gnu</a>/<a href='/guix/log/gnu/bootloader.scm?id=60562ec26bb512dbf9ea44de0df8b48f1f6e1880'>bootloader.scm</a></div><div class='content'><table class='list nowrap'><tr class='nohover'><th class='left'>Age</th><th class='left'>Commit message (<a href='/guix/log/gnu/bootloader.scm?id=60562ec26bb512dbf9ea44de0df8b48f1f6e1880&amp;showmsg=1'>Expand</a>)</th><th class='left'>Author</th></tr>
<tr><td><span title='2019-01-16 14:07:38 +0100'>2019-01-16</span></td><td><a href='/guix/commit/gnu/bootloader.scm?id=5f7467f046c3c1648cdf6eb81b3ec041bfc1f9bb'>bootloader: Remove deprecated 'device' field.</a><span class='msg-avail'>...</span></td><td>Ludovic Courtès</td></tr>
<tr><td><span title='2018-04-08 17:48:33 +0200'>2018-04-08</span></td><td><a href='/guix/commit/gnu/bootloader.scm?id=3c0128b035ae00462f1b0a4427d9525d750e5575'>discovery: Remove dependency on (guix ui).</a><span class='msg-avail'>...</span></td><td>Ludovic Courtès</td></tr>
<tr><td><span title='2017-08-28 09:56:33 +0200'>2017-08-28</span></td><td><a href='/guix/commit/gnu/bootloader.scm?id=9e1fe3d0c34e65a50ba93b5d7f7398503131c1ba'>bootloader: Emit warnings with 'warning'.</a><span class='msg-avail'>...</span></td><td>Ludovic Courtès</td></tr>
<tr><td><span title='2017-08-23 22:23:19 +0200'>2017-08-23</span></td><td><a href='/guix/commit/gnu/bootloader.scm?id=045ebb3e58d74c75f39ce47380045d3cd00359c3'>gnu: bootloader: Deprecate "device" field in favor of "target".</a><span class='msg-avail'>...</span></td><td>Andy Wingo</td></tr>
<tr><td><span title='2017-07-28 21:56:18 +0200'>2017-07-28</span></td><td><a href='/guix/commit/gnu/bootloader.scm?id=1975c754f487eb4933724ca3b116442d21ef8dd9'>bootloader: Use &lt;menu-entry&gt; for the bootloader side.</a><span class='msg-avail'>...</span></td><td>Danny Milosavljevic</td></tr>
<tr><td><span title='2017-06-08 16:29:29 +0200'>2017-06-08</span></td><td><a href='/guix/commit/gnu/bootloader.scm?id=8b22107e5d0bdeb300fb33b5a40aed2057a66b94'>bootloader: Use menu-entry to define custom bootloader entries.</a><span class='msg-avail'>...</span></td><td>Mathieu Othacehe</td></tr>
<tr><td><span title='2017-05-16 14:41:01 +0200'>2017-05-16</span></td><td><a href='/guix/commit/gnu/bootloader.scm?id=b09a8da4a2e50845a297e041762f3ff9e649c047'>bootloader: Add extlinux support.</a><span class='msg-avail'>...</span></td><td>Mathieu Othacehe</td></tr>