Replace free and return NULL with return mfree

src/basic/khash.c

@@ -127,9 +127,7 @@ khash* khash_unref(khash *h) {
 
         safe_close(h->fd);
         free(h->algorithm);
-        free(h);
-
-        return NULL;
+        return mfree(h);
 }
 
 int khash_dup(khash *h, khash **ret) {