builtin/apply: change die_on_unsafe_path() to check_unsafe_path()

builtin/apply: change die_on_unsafe_path() to check_unsafe_path()

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", die_on_unsafe_path() should return a negative
integer instead of calling die(), so while doing that let's change
its name to check_unsafe_path().

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