contrib/mod_ldap.c: fix SIGSEGV in mod_ldap:ldap_mod_init ()

The crash happens due to missing sentinel value in `pstrcat()`

```c
   feats = pstrcat(tmp_pool, feats, i != 0 ? ", " : "",
     api_info.ldapai_extensions[i]);
```

The change is to add sentinel to `pstrcat()` call.

Bug: #1027
Signed-off-by: Sergei Trofimovich <slyfox@gentoo.org>

Author: Sergei Trofimovich

contrib/mod_ldap.c

@@ -3218,7 +3218,7 @@ static int ldap_mod_init(void) {
 
         for (i = 0; api_info.ldapai_extensions[i]; i++) {
           feats = pstrcat(tmp_pool, feats, i != 0 ? ", " : "",
-            api_info.ldapai_extensions[i]);
+            api_info.ldapai_extensions[i], NULL);
           ldap_memfree(api_info.ldapai_extensions[i]);
         }