Handle null QualType better in Stmt::Profile

If the QualType is null, calling ASTContext::getCanonicalType on it will lead
to an assert.  This was found while testing a new use for Stmt::Profile, so
there is no test case for this.

llvm-svn: 296956

clang/lib/AST/StmtProfile.cpp

@@ -128,7 +128,7 @@ namespace {
     }
 
     void VisitType(QualType T) override {
-      if (Canonical)
+      if (Canonical && !T.isNull())
         T = Context.getCanonicalType(T);
 
       ID.AddPointer(T.getAsOpaquePtr());