Bug 1917665 - Return early on MaybeSomething if PromiseObj is null r=smaug

Promise::CreateInfallible allows not having PromiseObj but MaybeSomething still had an invalid assertion.

MaybeSomething eventually calls MaybeReject/Resolve which safely checks PromiseObj ultimately, but why go further when we can return early.

Differential Revision: https://phabricator.services.mozilla.com/D228465

dom/promise/Promise.h

@@ -423,7 +423,9 @@ class Promise : public SupportsWeakPtr {
 
   template <typename T>
   void MaybeSomething(T&& aArgument, MaybeFunc aFunc) {
-    MOZ_ASSERT(PromiseObj());  // It was preserved!
+    if (NS_WARN_IF(!PromiseObj())) {
+      return;
+    }
 
     AutoAllowLegacyScriptExecution exemption;
     AutoEntryScript aes(mGlobal, "Promise resolution or rejection");