merge: use return value of resolve_ref() to determine if HEAD is invalid

merge: use return value of resolve_ref() to determine if HEAD is invalid

resolve_ref() only updates "head" when it returns non NULL value (it
may update "head" even when returning NULL, but not in all cases).

Because "head" is not initialized before the call, is_null_sha1() is
not enough. Check also resolve_ref() return value.

Signed-off-by: Nguyễn Thái Ngọc Duy <pclouds@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>