virstring: Resolve Coverity FORWARD_NULL

Perhaps a false positive, but since Coverity doesn't understand the
relationship between the 'count' and the 'strings', rather than leave
the chance the on input 'strings' is NULL and causes a deref - just
check for it and return
Signed-off-by: NJohn Ferlan <jferlan@redhat.com>

src/util/virstring.c

@@ -198,6 +198,9 @@ virStringFreeListCount(char **strings,
 {
     size_t i;
 
+    if (!strings)
+        return;
+
     for (i = 0; i < count; i++)
         VIR_FREE(strings[i]);