target/cortex_m.c: vector_catch command checks if a target is examined

target/cortex_m.c: vector_catch command checks if a target is examined

If a target is not examined, command vector_catch crashes while accessing
the debug_ap NULL pointer.

maskisr and reset_config commands don't require this check.

Change-Id: I949b6f6e8b983327dd98fbe403735141f8f0b5d6
Signed-off-by: Daniel Trnka <daniel.trnka@gmail.com>
Reviewed-on: http://openocd.zylin.com/5813
Tested-by: jenkins
Reviewed-by: Tarek BOCHKATI <tarek.bouchkati@gmail.com>
Reviewed-by: Antonio Borneo <borneo.antonio@gmail.com>