gdbstub: don't fail on vCont; C04:0; c packets

gdbstub: don't fail on vCont; C04:0; c packets

The thread-id of 0 means any CPU but we then ignore the fact we find
the first_cpu in this case who can have an index of 0. Instead of
bailing out just test if we have managed to match up thread-id to a
CPU.

Otherwise you get:
  gdb_handle_packet: command='vCont;C04:0;c'
  put_packet: reply='E22'

The actual reason for gdb sending vCont;C04:0;c was fixed in a
previous commit where we ensure the first_cpu's tid is correctly
reported to gdb however we should still behave correctly next time it
does send 0.

Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Reviewed-by: Greg Kurz <groug@kaod.org>
Reviewed-by: Claudio Imbrenda <imbrenda@linux.vnet.ibm.com>
Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Message-Id: <20170712105216.747-5-alex.bennee@linaro.org>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>