mingw: do not crash on open(NULL, ...)

mingw: do not crash on open(NULL, ...)

fetch_and_setup_pack_index() apparently pass a NULL-pointer to
parse_pack_index(), which in turn pass it to check_packed_git_idx(),
which again pass it to open(). Since open() already sets errno
correctly for the NULL-case, let's just avoid the problematic strcmp.

[PT: squashed in fix for fopen which was missed first time round]

Acked-by: Johannes Sixt <redacted>
Signed-off-by: Erik Faye-Lund <redacted>
Signed-off-by: Pat Thoyts <redacted>