target: add valid smp target check

Check that the target is valid before calling any target functions.

Change-Id: I538fccc79d5ec89976e14beab02cb20490b299bb
Signed-off-by: Spencer Oliver <spen@spen-soft.co.uk>
Reviewed-on: http://openocd.zylin.com/766


Tested-by: jenkins
Reviewed-by: Freddie Chopin <freddie.chopin@gmail.com>