am: check return value of resolve_refdup before using hash

If resolve_refdup() fails it returns NULL and possibly leaves its hash
output parameter untouched.  Make sure to use it only if the function
succeeded, in order to avoid accessing uninitialized memory.

Signed-off-by: Rene Scharfe <[email protected]>
Signed-off-by: Junio C Hamano <[email protected]>

Author: rscharfe

builtin/am.c

@@ -2157,7 +2157,7 @@ static void am_abort(struct am_state *state)
 	am_rerere_clear();
 
 	curr_branch = resolve_refdup("HEAD", 0, curr_head.hash, NULL);
-	has_curr_head = !is_null_oid(&curr_head);
+	has_curr_head = curr_branch && !is_null_oid(&curr_head);
 	if (!has_curr_head)
 		hashcpy(curr_head.hash, EMPTY_TREE_SHA1_BIN);