Bug 1638331 - Expand the conditions for crashing in GetDWriteSystemFonts. r=jfkthame

We're not seeing any crashes so let's look for a null systemFonts
as well.

Differential Revision: https://phabricator.services.mozilla.com/D75546

gfx/2d/Factory.cpp

@@ -933,7 +933,7 @@ RefPtr<IDWriteFontCollection> Factory::GetDWriteSystemFonts(bool aUpdate) {
   RefPtr<IDWriteFontCollection> systemFonts;
   HRESULT hr =
       mDWriteFactory->GetSystemFontCollection(getter_AddRefs(systemFonts));
-  if (FAILED(hr)) {
+  if (FAILED(hr) || !systemFonts) {
     // only crash some of the time so those experiencing this problem
     // don't stop using Firefox
     if ((rand() & 0x3f) == 0) {