Fix crash caused by null callback in MdnsHostLocator

- Fixes issue where switching wifi networks while starting Mdns
  HostLocator would crash chrome.

Bug: chromium:900779
Test: end to end
Change-Id: Ide5e1bb53961dcd654aedadbffc6cf2a83c5d998
Reviewed-on: https://chromium-review.googlesource.com/c/1313331Reviewed-by: Bailey Berro <baileyberro@chromium.org>
Commit-Queue: jimmy gong <jimmyxgong@chromium.org>
Cr-Commit-Position: refs/heads/master@{#604753}

chrome/browser/chromeos/smb_client/discovery/mdns_host_locator.cc

@@ -59,14 +59,14 @@ void MDnsHostLocator::FindHosts(FindHostsCallback callback) {
     Reset();
   }
 
+  callback_ = std::move(callback);
+
   if (!(StartListening() && CreatePtrTransaction())) {
     LOG(ERROR) << "Failed to start MDnsHostLocator";
 
     FireCallback(false /* success */);
     return;
   }
-
-  callback_ = std::move(callback);
 }
 
 bool MDnsHostLocator::CreatePtrTransaction() {