object-file: convert 'switch' back to 'if'

object-file: convert 'switch' back to 'if'

This switch statement was recently added to make it clear that
unpack_loose_header() returns an enum value, not an int. This adds
complications for future developers if that enum gains new values, since
that developer would need to add a case statement to this switch for
little real value.

Instead, we can revert back to an 'if' statement, but make the enum
explicit by using "!= ULHR_OK" instead of assuming it has the numerical
value zero.

Co-authored-by: Junio C Hamano <gitster@pobox.com>
Signed-off-by: Derrick Stolee <derrickstolee@github.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>