gdb: use frame_id_p instead of comparing to null_frame_id in frame_info_ptr::reinflate

gdb: use frame_id_p instead of comparing to null_frame_id in frame_info_ptr::reinflate

The assertion

    gdb_assert (m_cached_id != null_frame_id);

is always true, as comparing equal to null_frame_id is always false
(it's the first case in frame_id::operator==, not sure why it's not this
way, but that's what it is).

Replace the comparison with a call to frame_id_p.

Approved-By: Tom Tromey <tom@tromey.com>
Change-Id: I93986e6a85ac56353690792552e5b3b4cedec7fb