[analyzer] Pointers-to-members are (currently) Locs, not NonLocs.

While we don't model pointers-to-members besides "null" and "non-null",
we were using Loc symbols for valid pointers and NonLoc integers for the
null case. This hit the assert committed in r185401.

Fixed by using a true (Loc) null for null member pointers.

llvm-svn: 185444