Avoid getting an argument of allocation function if it does not exist.

This is a fix to PR12778: in erroneous code an allocation function
can be declared with no arguments, quering the first argument in this case
causes assertion violation.


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@190751 91177308-0d34-0410-b5e6-96231b3b80d8