Bug 1709466: Don't prepare for process change when already discarded. r=nika

If a <browser> is removed in the middle of a process change, the previous
DocShell may be torn down while it's still expecting a process change. There's
really nothing more to do in that case, though, so we can just skip the
PrepareForProcessChange call rather than asserting.

Differential Revision: https://phabricator.services.mozilla.com/D118514

docshell/base/nsDocShell.cpp

@@ -4513,7 +4513,7 @@ nsDocShell::Destroy() {
     GetSessionHistory()->EvictLocalContentViewers();
   }
 
-  if (mWillChangeProcess) {
+  if (mWillChangeProcess && !mBrowsingContext->IsDiscarded()) {
     mBrowsingContext->PrepareForProcessChange();
   }