(Ada) crash connecting to TSIM simulator

(Ada) crash connecting to TSIM simulator

Connecting to a TSIM simulator over the remote protocol causes GDB
to crash with the following failed assertion:

    (gdb) tar remote :1234
    Remote debugging using :1234
    /[...]/gdb/ravenscar-thread.c:182: internal-error: ravenscar_update_inferior_ptid: Assertion `!is_ravenscar_task (inferior_ptid)' failed.
    A problem internal to GDB has been detected,
    further debugging may prove unreliable.
    Quit this debugging session? (y or n) y

What happens is the following. Upon connection to the target, GDB
sends a 'qfThreadInfo' query, which is the query asking the target
for the ID of the first thread, and TSIM replies 'm0':

    Sending packet: $qfThreadInfo#bb...Ack
    Packet received: m0

As a result of this, GDB takes the '0' as the TID, and because of it,
constructs a ptid whose value is {42000, 0, 0}. This trips our
!is_ravenscar_task check, because all it does to identify threads
corresponding to ravenscar tasks is that their lwp is null, because
that's how we construct their ptid.

But this is unfortunatly not sufficient when debugging with TSIM,
because the thread ID that TSIM returns causes the creation of
a ptid whose lwp is zero, which matches the current identification
scheme and yet is clearly not a ravenscar task.

The fix is to also make sure that the ptid's tid field is nonzero.

gdb/ChangeLog:

        * ravenscar-thread.c (is_ravenscar_task): Also verify that
        the ptid's TID is nonzero.