Address xedin's review feedback

Author: naveen-seth

lib/Sema/CSDiagnostics.cpp

@@ -2142,9 +2142,9 @@ bool TrailingClosureAmbiguityFailure::diagnoseAsNote() {
       return false;
 
     const ParameterList *paramList = callee->getParameters();
-    if (paramList->getArray().empty())
+    if (!paramList || paramList->size() == 0)
       return false;
-    const ParamDecl *param = paramList->getArray().back();
+    const ParamDecl *param = paramList->back();
 
     // Soundness-check that the trailing closure corresponds to this parameter.
     if (!param->hasInterfaceType() ||