tcl/drscan: handle invalid syntax with a conditional, not assert

When "drscan" command is used improperly, such as in:
drscan stm32f1x.cpu -endstate drpause
there're no fields to scan, and so the assert leads to a
segfault. This should be treated like any other syntax error instead.

Change-Id: Id1743f5d641038e1e3754c6f3097aabc5d1916b9
Signed-off-by: Paul Fertser <fercerpav@gmail.com>
Reviewed-on: http://openocd.zylin.com/1927


Tested-by: jenkins
Reviewed-by: Spencer Oliver <spen@spen-soft.co.uk>