[Sema] Add missing null check for getAttachedResultBuilder

I haven't been able to come up with a test case, but it seems like
it's possible for the attribute to be invalidated during the call
to `visitCustomAttr`, in which case `getAttachedResultBuilder` can
return `nullptr`.

rdar://118642163

Author: hamishknight

lib/Sema/TypeCheckAttr.cpp

@@ -4638,7 +4638,7 @@ void AttributeChecker::visitCustomAttr(CustomAttr *attr) {
 
     // Complain if this isn't the primary result-builder attribute.
     auto attached = decl->getAttachedResultBuilder();
-    if (attached != attr) {
+    if (attached && attached != attr) {
       diagnose(attr->getLocation(), diag::result_builder_multiple,
                isa<ParamDecl>(decl));
       diagnose(attached->getLocation(), diag::previous_result_builder_here);