Adapt loplugin:derefnullptr to old Clang versions

...where prior to r183883 "Implement core issue 903: only integer literals with
value 0 and prvalues of type std::nullptr_t are null pointer constants from
C++11 onwards," Expr::isNullPointerConstant with NPC_NeverValueDependent could
go into an llvm_unreachable case.

Change-Id: I29cf093f18ece4cd83fd759e30f72c2a71f69554
Avoid needless truncation to int

Change-Id: Ibb9088e73e3e48952ce7f2f4ec42392f7633e8a7