builtin/merge.c: free "&buf" on "Your local changes..." error

Plug a memory leak introduced in [1], since that change didn't follow
the "goto done" pattern introduced in [2] we'd leak the "&buf" memory.

1. e4cdfe8 (merge: abort if index does not match HEAD for trivial
   merges, 2022-07-23)
2. d5a35c1 (Copy resolve_ref() return value for longer use,
   2011-11-13)

Signed-off-by: Ævar Arnfjörð Bjarmason <[email protected]>
Signed-off-by: Junio C Hamano <[email protected]>

Author: avar

builtin/merge.c

@@ -1618,7 +1618,8 @@ int cmd_merge(int argc, const char **argv, const char *prefix)
 				error(_("Your local changes to the following files would be overwritten by merge:\n  %s"),
 				      sb.buf);
 				strbuf_release(&sb);
-				return 2;
+				ret = 2;
+				goto done;
 			}
 
 			/* See if it is really trivial. */