[UBsan] Skip -fsanitize=vptr instrumentations when the pointer value is null.

Otherwise -fsanitize=vptr causes the program to crash when it downcasts
a null pointer.

Reviewed in http://reviews.llvm.org/D4412.
Patch by Byoungyoung Lee!


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@213393 91177308-0d34-0410-b5e6-96231b3b80d8