builtin/apply: make gitdiff_verify_name() return void

builtin/apply: make gitdiff_verify_name() return void

As the value returned by gitdiff_verify_name() is put into the
same variable that is passed as a parameter to this function,
it is simpler to pass the address of the variable and have
gitdiff_verify_name() change the variable itself.

This also makes it possible to later have this function return
-1 instead of die()ing in case of error.

Reviewed-by: Stefan Beller <sbeller@google.com>
Signed-off-by: Christian Couder <chriscool@tuxfamily.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>