Since p_cansee will end up dereferencing p_ucred, don't check for p_ucred

Since p_cansee will end up dereferencing p_ucred, don't check for p_ucred
equal to NULL several times later.  p_ucred "should probably not" be NULL
if the process isn't PRS_NEW anyway.  This is strongly reinforced by the fact
that we don't see frequent crashes here.  Remove the checks after p_cansee and
add a KASSERT right before it.

Found by:	Coverity Prevent (tm)

Also trim one nearby trailing space.