target: make it absolutely clear that no null pointers are accepted

target: make it absolutely clear that no null pointers are accepted

there are no comments about what the rules w.r.t. null pointers
are and it is inconsistent.

It's simply a bug in the app if we ask about the properties of a null
pointer w.r.t. what kind of target it is. It's equally wrong to say
that it is an arm target as that it isn't an arm target.

Change-Id: I0925a6a5c8b38e594ffa7c3ca4390487b5e9b718
Signed-off-by: Øyvind Harboe <oyvind.harboe@zylin.com>
Reviewed-on: http://openocd.zylin.com/168
Tested-by: jenkins
Reviewed-by: Andreas Fritiofson <andreas.fritiofson@gmail.com>