Check for NULL child expressions before visiting them, as the first

thing the visit does is dyn_cast<>, which leads to a nasty segfault.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@125993 91177308-0d34-0410-b5e6-96231b3b80d8

include/clang/AST/EvaluatedExprVisitor.h

@@ -72,7 +72,8 @@ public:
   /// expression, assuming they are all potentially evaluated.
   void VisitStmt(Stmt *S) {
     for (Stmt::child_range C = S->children(); C; ++C)
-      this->Visit(*C);
+      if (*C)
+        this->Visit(*C);
   }
 };