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
diff --git a/clang/lib/Sema/SemaTemplateInstantiate.cpp b/clang/lib/Sema/SemaTemplateInstantiate.cpp
index a7d03dd..e8f1dcc 100644
--- a/clang/lib/Sema/SemaTemplateInstantiate.cpp
+++ b/clang/lib/Sema/SemaTemplateInstantiate.cpp
@@ -2892,7 +2892,7 @@
       unsigned i = PV->getFunctionScopeIndex();
       // This parameter might be from a freestanding function type within the
       // function and isn't necessarily referring to one of FD's parameters.
-      if (FD->getParamDecl(i) == PV)
+      if (i < FD->getNumParams() && FD->getParamDecl(i) == PV)
         return FD->getCanonicalDecl()->getParamDecl(i);
     }
   }