Don't crash checking a format string if one of the arguments is invalid.

Previously, we would ask for the SourceLocation of an argument even if
it were NULL (i.e. if Sema resulted in an ExprError trying to build it).

<rdar://problem/11890818>

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@160515 91177308-0d34-0410-b5e6-96231b3b80d8