Bug 1810903, return early from LoadErrorPage if the docshell is going away, r=peterv

We'd return early anyhow later in nsDocShell::InternalLoad.
The patch is based on code inspection. If one is closing the window while printing, we could get to this state, among other cases.

Differential Revision: https://phabricator.services.mozilla.com/D168368

docshell/base/nsDocShell.cpp

@@ -3956,7 +3956,9 @@ nsresult nsDocShell::LoadErrorPage(nsIURI* aURI, const char16_t* aURL,
                                    const char16_t* aDescription,
                                    const char* aCSSClass,
                                    nsIChannel* aFailedChannel) {
-  MOZ_ASSERT(!mIsBeingDestroyed);
+  if (mIsBeingDestroyed) {
+    return NS_ERROR_NOT_AVAILABLE;
+  }
 
 #if defined(DEBUG)
   if (MOZ_LOG_TEST(gDocShellLog, LogLevel::Debug)) {