receive-pack: fix crash on out-of-namespace symref

receive-pack: fix crash on out-of-namespace symref

`check_aliased_update_internal()` detects when a symbolic ref and its
target are being updated in the same push. It does this by building a
list of ref names without the optional namespace prefix. When a symbolic
ref within a namespace points to a ref outside the namespace,
`strip_namespace()` returns NULL which leads to a segfault.

A NULL check preventing this particular issue was repurposed in
ded8393610. Rather than reintroducing it, we can instead build a list of
fully qualified ref names. This prevents the crash, preserves the
consistency check from da3efdb17b, and allows updates to all symbolic
refs.

Signed-off-by: Troels Thomsen <troels@thomsen.io>
Signed-off-by: Junio C Hamano <gitster@pobox.com>