Prevent false positives when freeing a NULL pointer with MALLOC_DEBUG enabled.

git-svn-id: https://origsvn.digium.com/svn/asterisk/branches/1.4@203230 65c4cc65-6c06-0410-ace0-fbb531ad65f3

main/astmm.c

@@ -157,10 +157,16 @@ static inline size_t __ast_sizeof_region(void *ptr)
 
 static void __ast_free_region(void *ptr, const char *file, int lineno, const char *func)
 {
-	int hash = HASH(ptr);
+	int hash;
 	struct ast_region *reg, *prev = NULL;
 	unsigned int *fence;
 
+	if (!ptr) {
+		return;
+	}
+
+	hash = HASH(ptr);
+
 	ast_mutex_lock(&reglock);
 	for (reg = regions[hash]; reg; reg = reg->next) {
 		if (reg->data == ptr) {