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 <l.s.r@web.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

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);