Make sure CallExpr::getLocStart doesn't segfault

Summary:
When the CallExpr passed to Sema::ConvertArgumentsForCall has all default parameters, and the number of actual arguments passed is zero, this function will segfault in the call to Call->getLocStart() if the Callee has an invalid getLocStart(), the reason being that since ConvertArgumentsForCall has set the correct number of arguments, but has not filled them in yet, getLocStart() will try to access the first (not yet existent) argument and thus segfaults.

This fixes that by making getLocStart return an invalid source location if the queried argument is NULL rather than segfaulting.

Reviewers: rnk

Reviewed By: rnk

Subscribers: cfe-commits

Differential Revision: http://reviews.llvm.org/D4917