builtin/apply: make find_header() return -128 instead of die()ing

builtin/apply: make find_header() return -128 instead of die()ing

To libify `git apply` functionality we have to signal errors to the
caller instead of die()ing.

To do that in a compatible manner with the rest of the error handling
in builtin/apply.c, let's make find_header() return -128 instead of
calling die().

We could make it return -1, unfortunately find_header() already
returns -1 when no header is found.

Signed-off-by: Christian Couder <redacted>
Signed-off-by: Junio C Hamano <redacted>