list-objects-filter-options: move error check up

list-objects-filter-options: move error check up

Move the check that filter_options->choice is set to higher in the call
stack. This can only be set when the gentle parse function is called
from one of the two call sites.

This is important because in an upcoming patch this may or may not be an
error, and whether it is an error is only known to the
parse_list_objects_filter function.

Signed-off-by: Matthew DeVore <matvore@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>