Fix BodySnifferURLLoader::Cancel() crash on invalid WeakPtr dereference (#34003)

Add a null check for throttle_ before dereferencing in Cancel(),
consistent with existing guards in OnComplete() and CompleteSniffing().

Resolves: https://github.com/brave/brave-browser/issues/52931

components/body_sniffer/body_sniffer_url_loader.cc

@@ -450,6 +450,10 @@ void BodySnifferURLLoader::ForwardBodyToClient() {
 }
 
 void BodySnifferURLLoader::Cancel() {
+  if (!throttle_) {
+    Abort();
+    return;
+  }
   throttle_->Cancel();
 }