get_local_heads(): do not return random pointer if there is no head

it's silly to do this:

mkdir foo && cd foo && git init && git push somewhere.git

but segfault should not happen even in that case.

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