Fix a null pointer dereference when no DNS search path is configured.
This showed up as a segfault while running the gsasl test suite.

Taken from upstream:
https://github.com/krb5/krb5/commit/cd82bf377e7fad2409c76bf8b241920692f34fda

diff --git a/src/lib/krb5/os/dnsglue.c b/src/lib/krb5/os/dnsglue.c
index e35ca9d76c..0cd213fdd7 100644
--- a/src/lib/krb5/os/dnsglue.c
+++ b/src/lib/krb5/os/dnsglue.c
@@ -91,7 +91,7 @@ static int initparse(struct krb5int_dns_state *);
 #define DECLARE_HANDLE(h) struct __res_state h
 #define INIT_HANDLE(h) (memset(&h, 0, sizeof(h)), res_ninit(&h) == 0)
 #define SEARCH(h, n, c, t, a, l) res_nsearch(&h, n, c, t, a, l)
-#define PRIMARY_DOMAIN(h) strdup(h.dnsrch[0])
+#define PRIMARY_DOMAIN(h) ((h.dnsrch[0] == NULL) ? NULL : strdup(h.dnsrch[0]))
 #if HAVE_RES_NDESTROY
 #define DESTROY_HANDLE(h) res_ndestroy(&h)
 #else
@@ -104,7 +104,8 @@ static int initparse(struct krb5int_dns_state *);
 #define DECLARE_HANDLE(h)
 #define INIT_HANDLE(h) (res_init() == 0)
 #define SEARCH(h, n, c, t, a, l) res_search(n, c, t, a, l)
-#define PRIMARY_DOMAIN(h) strdup(_res.defdname)
+#define PRIMARY_DOMAIN(h) \
+    ((_res.defdname == NULL) ? NULL : strdup(_res.defdname))
 #define DESTROY_HANDLE(h)
 
 #endif
f13c64b72a2dc5f0d30a259'>tree</a><a href='/guix/commit/gnu/tests.scm?id=116bfb3cd0c55b1c4f13c64b72a2dc5f0d30a259'>commit</a><a href='/guix/diff/gnu/tests.scm?id=116bfb3cd0c55b1c4f13c64b72a2dc5f0d30a259'>diff</a></td><td class='form'><form class='right' method='get' action='/guix/log/gnu/tests.scm'>
<input type='hidden' name='id' value='116bfb3cd0c55b1c4f13c64b72a2dc5f0d30a259'/><select name='qt'>
<option value='grep'>log msg</option>
<option value='author'>author</option>
<option value='committer'>committer</option>
<option value='range'>range</option>
</select>
<input class='txt' type='search' size='10' name='q' value=''/>
<input type='submit' value='search'/>
</form>
</td></tr></table>
<div class='path'>path: <a href='/guix/log/?id=116bfb3cd0c55b1c4f13c64b72a2dc5f0d30a259'>root</a>/<a href='/guix/log/gnu?id=116bfb3cd0c55b1c4f13c64b72a2dc5f0d30a259'>gnu</a>/<a href='/guix/log/gnu/tests.scm?id=116bfb3cd0c55b1c4f13c64b72a2dc5f0d30a259'>tests.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/tests.scm?id=116bfb3cd0c55b1c4f13c64b72a2dc5f0d30a259&amp;showmsg=1'>Expand</a>)</th><th class='left'>Author</th></tr>
<tr><td><span title='2023-11-05 23:23:23 +0100'>2023-11-05</span></td><td><a href='/guix/commit/gnu/tests.scm?id=0c695bbf8dbd5a5c1224f331a78b6316bc16e4d1'>tests: Honor system and target when lowering &lt;system-test&gt;.</a><span class='msg-avail'>...<span class='msg-tooltip'>* gnu/tests.scm (compile-system-test): Wrap ‘system-test-value’ call in
‘mparameterize’.

Change-Id: I4be28913a86f43059b0886ad2fcf81a9c699b730
</span></span></td><td>Ludovic Courtès</td></tr>
<tr><td><span title='2023-04-21 16:16:38 +0200'>2023-04-21</span></td><td><a href='/guix/commit/gnu/tests.scm?id=a09c7da8f8d8e732f969cf0a09aaa78f87032ab1'>tests: Fork and exec a new Guile for the marionette REPL.</a><span class='msg-avail'>...<span class='msg-tooltip'>By merely forking PID 1, details from PID 1 (shepherd) would leak into
the marionette process, such as the set of modules in scope and state
inherited from the shepherd process (&lt;service&gt; instances, fibers,
etc.).  Running a fresh Guile instance avoids that.

* gnu/tests.scm (marionette-program): New procedure.
(marionette-shepherd-service): Change 'start' to use
'make-forkexec-constructor', and run the result of 'marionette-program'.
</span></span></td><td>Ludovic Courtès</td></tr>