fix "git apply --index ..." not to deref NULL

fix "git apply --index ..." not to deref NULL

I noticed this when "git am CORRUPTED" unexpectedly failed with an
odd diagnostic, and even removed one of the files it was supposed
to have patched.

Reproduce with any valid old/new patch from which you have removed
the "+++ b/FILE" line.  You'll see a diagnostic like this

    fatal: unable to write file '(null)' mode 100644: Bad address

and you'll find that FILE has been removed.

The above is on glibc-based systems.  On other systems, rather than
getting "null", you may provoke a segfault as git tries to
dereference the NULL file name.

Signed-off-by: Jim Meyering <meyering@redhat.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>