lib: Don't access configuration if none is present

Commit e457d5ef20 adds ability to pass the
default URI using the client configuration file. If the file is not
present, it still accesses the NULL config object causing a segfault.

Caught running "make check".

src/libvirt.c

@@ -1085,7 +1085,8 @@ virConnectOpenResolveURIAlias(virConfPtr conf,
 
     *uri = NULL;
 
-    if ((value = virConfGetValue(conf, "uri_aliases")))
+    if (conf &&
+        (value = virConfGetValue(conf, "uri_aliases")))
         ret = virConnectOpenFindURIAliasMatch(value, alias, uri);
     else
         ret = 0;