Sema: Don't dyn_cast a null pointer in CheckUsingDeclQualifier

This code was written with the intent that a pointer could be null but
we dyn_cast'd it anyway.  Change the dyn_cast to a dyn_cast_or_null.

This fixes PR21933.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@224411 91177308-0d34-0410-b5e6-96231b3b80d8