Do not start discover timer if there are no valid interfaces

Previously, if there were no valid NetworkInterfaces we would exit out of
NetBios discovery, but the discovery timer would still be set causing a
moved-from callback to be run twice.

The crash was especially subtle becuase it required having no valid
network interfaces, which is uncommon because both Wifi and Ethernet are
valid interfaces.

Bug: chromium:882972
Test: verified manually on Elm device
Change-Id: I2e7a28aded718845a08669f464b60bba2bbc6174
Reviewed-on: https://chromium-review.googlesource.com/1226364
Commit-Queue: Zentaro Kavanagh <zentaro@chromium.org>
Reviewed-by: Zentaro Kavanagh <zentaro@chromium.org>
Cr-Commit-Position: refs/heads/master@{#591433}

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

@@ -75,6 +75,7 @@ void NetBiosHostLocator::FindHosts(FindHostsCallback callback) {
     // no valid interfaces.
     running_ = false;
     std::move(callback_).Run(false /* success */, results_);
+    return;
   }
 
   timer_->Start(FROM_HERE,