Fix drd assert failure for monitor cmd execution when all threads blocked in syscall

When all threads are blocked in a syscall, DRD_(thread_get_running_tid)()
returns (or can return?) a "valid" DrdThreadId (probably the last one
that was running ?).
However, in such a situation, VG_(get_running_tid)() returns 0 (as effectively
there is no thread running).

This discrepancy (drd_tid "valid" and "vg_tid" invalid) can (probably) only
happen with gdb monitor commands, as a thread can (normally) execute a
client request only when it is running.

However, vgdb can "force" the invocation of the gdb monitor client request,
even if no thread is running.
In such a case, the assert should not verify that drd_tid and vg_tid are
consistent.

To reproduce the problem:
  ./vg-in-place --tool=drd sleep 60
and in another window:
  ./coregrind/vgdb help
which then gives
  ...
  drd: drd_clientreq.c:84 (handle_client_request): Assertion 'DRD_(VgThreadIdToDrdThreadId)(vg_tid) == drd_tid' failed.
  ==4208==    at 0x3801DE1D: report_and_quit (m_libcassert.c:260)
  ==4208==    by 0x3801E034: vgPlain_assert_fail (m_libcassert.c:340)
  ==4208==    by 0x380026CC: handle_client_request (drd_clientreq.c:84)
  ==4208==    by 0x3806EE8F: handle_gdb_monitor_command (server.c:490)
  ==4208==    by 0x3806F07A: handle_query (server.c:580)
  ==4208==    by 0x3806FDB2: server_main (server.c:915)
  ==4208==    by 0x3806B01C: call_gdbserver (m_gdbserver.c:706)
  ==4208==    by 0x3806B8F2: vgPlain_invoke_gdbserver (m_gdbserver.c:892)
  ...




git-svn-id: svn://svn.valgrind.org/valgrind/trunk@13765 a5019735-40e9-0310-863c-91ae7b9d1cf9