Fix a crash I introduced in r159212.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@159279 91177308-0d34-0410-b5e6-96231b3b80d8
diff --git a/lib/AST/DeclCXX.cpp b/lib/AST/DeclCXX.cpp
index a8aabb6..4de157e 100644
--- a/lib/AST/DeclCXX.cpp
+++ b/lib/AST/DeclCXX.cpp
@@ -1290,7 +1290,7 @@
   // Lookup doesn't work for destructors, so handle them separately.
   if (isa<CXXDestructorDecl>(this)) {
     CXXMethodDecl *MD = RD->getDestructor();
-    if (recursivelyOverrides(MD, this))
+    if (MD && recursivelyOverrides(MD, this))
       return MD;
     return NULL;
   }