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
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<tr><td><span title='2020-09-17 14:10:42 +0200'>2020-09-17</span></td><td><a href='/guix/commit/etc/openrc?id=cabac732deffafc796537658a6c33c6c4aec821e'>guix-install.sh: Support OpenRC.</a><span class='msg-avail'>...</span></td><td>Morgan Smith</td></tr>
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<tr><td><span title='2021-06-16 12:13:39 +0200'>2021-06-16</span></td><td><a href='/guix/commit/etc/completion/bash?id=dc3ba8c83602d69294e21d1b0c066f0d89890b56'>bash completion: Complete options for the right command.</a><span class='msg-avail'>...</span></td><td>Tobias Geerinckx-Rice</td></tr>
<tr><td><span title='2021-06-16 12:13:39 +0200'>2021-06-16</span></td><td><a href='/guix/commit/etc/completion/bash?id=80a17aae7991c6df061a98bb71734485f4ca17e2'>bash completion: Really support subcommands.</a><span class='msg-avail'>...</span></td><td>Tobias Geerinckx-Rice</td></tr>
<tr><td><span title='2020-12-04 23:45:08 +0100'>2020-12-04</span></td><td><a href='/guix/commit/etc/completion/bash?id=611ae310f4ca1d93600c280e940a69b07e20c350'>bash completion: Complete file names after '-f' and '-l'.</a><span class='msg-avail'>...</span></td><td>Ludovic Courtès</td></tr>
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<tr><td><span title='2020-09-21 23:33:49 +0200'>2020-09-21</span></td><td><a href='/guix/commit/etc/completion/bash?id=92951bf46c42021f6f6521839c9c4ff2f88b8386'>bash completion: Complete file names after 'guix package -p'.</a><span class='msg-avail'>...</span></td><td>Fulbert</td></tr>
<tr><td><span title='2020-09-10 12:27:24 +0200'>2020-09-10</span></td><td><a href='/guix/commit/etc/completion/bash?id=2ba81afcc204024255ce548e2c4cc4aed5541093'>bash completion: Restore completion for 'guix system', etc.</a><span class='msg-avail'>...</span></td><td>Ludovic Courtès</td></tr>
<tr><td><span title='2020-06-27 23:42:20 +0200'>2020-06-27</span></td><td><a href='/guix/commit/etc/completion/bash?id=4b4f890cb03d7c5b958fe08553dfad2f39a1d4f2'>bash completion: Complete 'guix environment' properly.</a><span class='msg-avail'>...</span></td><td>Ludovic Courtès</td></tr>
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<tr><td><span title='2020-02-14 17:49:26 +0100'>2020-02-14</span></td><td><a href='/guix/commit/etc/completion/bash?id=11415d35064cdba5cec1139aede18099cfa14547'>guix build: Add '--manifest' option.</a><span class='msg-avail'>...</span></td><td>Marius Bakke</td></tr>
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