PR23057: Fix assertion `Val && "isa<> used on a null pointer"' on invalid for-range expression.

Fix the issue discovered by fuzzing (PR23057, comment 18) by handling nullptr in Sema::ActOnCXXForRangeDecl 
and correct delayed typos in for-range expression before calling Sema::ActOnCXXForRangeStmt. Also fixes PR26288.

Differential Revision: http://reviews.llvm.org/D16630

llvm-svn: 259532