lib: add internal bt_field_sequence_get_int_length()

lib: add internal bt_field_sequence_get_int_length()

This new internal helper returns a sequence field's length as an
`int64_t` value instead of returning the integer field. Returns a
negative value on error or if the sequence fields's length field is not
set.

Signed-off-by: Philippe Proulx <eeppeliteloop@gmail.com>