Check if channel is closed.

If our event channel is closed, our last read results in nil.  This
patch makes bridgestrap check for this condition before accessing the
result's struct members.

tor.go

@@ -222,6 +222,11 @@ func (c *TorContext) TestBridgeLines(bridgeLines []string) *TestResult {
 	for {
 		select {
 		case ev := <-c.eventChan:
+			// Our channel is closed.
+			if ev == nil {
+				result.Error = "test aborted because bridgestrap is shutting down"
+				return result
+			}
 			for _, line := range ev.RawLines {
 				for bridgeLine, parser := range eventParsers {
 					// Skip bridges that are done testing.