Check for NULL ptr in malloc_usable_size().

Check for NULL ptr in malloc_usable_size(), rather than just asserting
that ptr is non-NULL.  This matches behavior of other implementations
(e.g., glibc and tcmalloc).

src/jemalloc.c

@@ -1230,10 +1230,8 @@ je_malloc_usable_size(const void *ptr)
 
 	if (config_ivsalloc)
 		ret = ivsalloc(ptr);
-	else {
+	else
-		assert(ptr != NULL);
+		ret = (ptr != NULL) ? isalloc(ptr) : 0;
-		ret = isalloc(ptr);
-	}
 
 	return (ret);
 }