Fix crash when command arg is missing in faas/taas/tfaas commands.

Fix crash when command arg is missing in faas/taas/tfaas commands.

GDB crashes when doing:
  (gdb) faas
  Aborted

Do the needed check to avoid crashing.

gdb/ChangeLog
2019-12-06  Philippe Waroquiers  <philippe.waroquiers@skynet.be>
	* stack.c (faas_command): Check a command is provided.
	* thread.c (taas_command, tfaas_command): Likewise.

gdb/testsuite/ChangeLog
2019-12-06  Philippe Waroquiers  <philippe.waroquiers@skynet.be>

	* gdb.threads/pthreads.exp: Test taas and tfaas without command.
	* gdb.base/frameapply.exp: Test faas without command.