fetch-pack: disregard invalid pack lockfiles

fetch-pack: disregard invalid pack lockfiles

9da69a6539 (fetch-pack: support more than one pack lockfile, 2020-06-10)
started to use a string_list for pack lockfile names instead of a single
string pointer.  It removed a NULL check from transport_unlock_pack() as
well, which is the function that eventually deletes these lockfiles and
releases their name strings.

index_pack_lockfile() can return NULL if it doesn't like the contents it
reads from the file descriptor passed to it.  unlink(2) is declared to
not accept NULL pointers (at least with glibc).  Undefined Behavior
Sanitizer together with Address Sanitizer detects a case where a NULL
lockfile name is passed to unlink(2) by transport_unlock_pack() in t1060
(make SANITIZE=address,undefined; cd t; ./t1060-object-corruption.sh).

Reinstate the NULL check to avoid undefined behavior, but put it right
at the source, so that the number of items in the string_list reflects
the number of valid lockfiles.

Signed-off-by: René Scharfe <l.s.r@web.de>
Reviewed-by: Taylor Blau <me@ttaylorr.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>