nss-myhostname: copy first result to preallocated buffer

Fixes a segfault in nscd when using nss-myhostname.

Nscd expects that an NSS module's gethostbyname4_r function returns
its first result in the pre-allocated gaih_addrtuple denoted by **pat.
(See nscd/aicache.c in the Glibc sources.)  However, nss-myhostname
doesn't fill in **pat but allocates the first result in ‘buffer’, then
sets *pat.  So nscd crashes (e.g. when running ‘getent ahosts
my-machine’).

Hard to tell if this is a bug in nscd, since there doesn't seem to be
a proper API spec for gethostbyname4_r.  But in any case, this patch
fixes the crash by copying the first result to **pat.

src/nss-myhostname/nss-myhostname.c

@@ -176,7 +176,11 @@ enum nss_status _nss_myhostname_gethostbyname4_r(
         /* Verify the size matches */
         assert(idx == ms);
 
-        *pat = r_tuple_prev;
+        /* Nscd expects us to store the first record in **pat. */
+        if (*pat)
+                **pat = *r_tuple_prev;
+        else
+                *pat = r_tuple_prev;
 
         if (ttlp)
                 *ttlp = 0;