Merge branch 'rs/checkout-am-fix-unborn'

A few codepaths in "checkout" and "am" working on an unborn branch
tried to access an uninitialized piece of memory.

* rs/checkout-am-fix-unborn:
am: check return value of resolve_refdup before using hash
checkout: check return value of resolve_refdup before using hash

diff --cc builtin/am.c
Simple merge

diff --cc builtin/checkout.c
Simple merge