Fix potential nullptr dereference.

Bug: 21597073

We must use dyn_cast_or_null<> instead of dyn_cast<>, because we aren't
guaranteed to always be passed a NamedDecl.

Change-Id: I7b00eacc639a688f8e2fdbeea80b87a01372e232
(cherry picked from commit 33ea573b6df7b7fe48d2b68d4c479f33082e3c0d)
1 file changed