Check i < FD->getNumParams() before querying

Summary:
As was already stated in a previous comment, the parameter isn't
necessarily referring to one of the DeclContext's parameter. We
should check the index is within the range to avoid out-of-boundary
access.

Reviewers: gribozavr, rsmith, lebedev.ri

Reviewed By: gribozavr, rsmith

Subscribers: lebedev.ri, cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D60055

Patch by Violet.

llvm-svn: 358134