Bug 1579492 - Check if nullptr in DarwinGamepadService::ReportChangedCallback(). r=baku

Differential Revision: https://phabricator.services.mozilla.com/D45135

--HG--
extra : moz-landing-system : lando

dom/gamepad/cocoa/CocoaGamepad.cpp

@@ -339,7 +339,7 @@ void DarwinGamepadService::DeviceRemoved(IOHIDDeviceRef device) {
 void DarwinGamepadService::ReportChangedCallback(
     void* context, IOReturn result, void* sender, IOHIDReportType report_type,
     uint32_t report_id, uint8_t* report, CFIndex report_length) {
-  if (report_type == kIOHIDReportTypeInput) {
+  if (context && report_type == kIOHIDReportTypeInput) {
     reinterpret_cast<Gamepad*>(context)->ReportChanged(report, report_length);
   }
 }