su_alloc.c: su_free() handles NULL gracefully

Patch by Michael Jerris.

darcs-hash:20080128142703-65a35-3d97fbf5fa5f720000b3e3d81533db954763702d.gz

libsofia-sip-ua/su/su_alloc.c

@@ -780,7 +780,10 @@ void *su_alloc(su_home_t *home, isize_t size)
  */
 void su_free(su_home_t *home, void *data)
 {
-  if (home && data) {
+  if (!data)
+    return;
+
+  if (home) {
     su_alloc_t *allocation;
     su_block_t *sub = MEMLOCK(home);