remote packet sent after Ravenscar inferior exited

remote packet sent after Ravenscar inferior exited

When debugging a program using the Ravenscar profile, the debugger
sometimes tries to send the following packet to the remote after
the inferior exited.

     (gdb) c
     Continuing.
     [...]
     Sending packet: $vCont;c:1#13...Ack
     Packet received: W00
     Sending packet: $Hg1#e0...putpkt: write failed: Broken pipe.

As the inferior exited, the remote has already disconnected, and thus
the operation fails.

The reason why GDB sends the package is because the ravenscar-thread
module tries to updates the list of threads.  But this doesn't make
sense, since the program has exited.  This patch fixes it.

gdb/ChangeLog:

        * ravenscar-thread.c (ravenscar_wait): Only update the list
        of threads and inferior_ptid if the inferior is still alive.