zmalloc fix, return NULL or real malloc failure

zmalloc.c

@@ -36,6 +36,7 @@ static size_t used_memory = 0;
 void *zmalloc(size_t size) {
     void *ptr = malloc(size+sizeof(size_t));
 
+    if (!ptr) return NULL;
     *((size_t*)ptr) = size;
     used_memory += size+sizeof(size_t);
     return (char*)ptr+sizeof(size_t);