rpclient: fix masked error causing crash after max retries

This commit fixes the error that is masked inside the for loop's scope.
Previously after max retries the error didn't leave the for scope and
therefore httpResponse remained nil which in turn resulted in a crash.

rpcclient/infrastructure.go

@@ -776,8 +776,10 @@ func (c *Client) handleSendPostMessage(jReq *jsonRequest) {
 
 	tries := 10
 	for i := 0; i < tries; i++ {
+		var httpReq *http.Request
+
 		bodyReader := bytes.NewReader(jReq.marshalledJSON)
-		httpReq, err := http.NewRequest("POST", url, bodyReader)
+		httpReq, err = http.NewRequest("POST", url, bodyReader)
 		if err != nil {
 			jReq.responseChan <- &Response{result: nil, err: err}
 			return
@@ -815,6 +817,14 @@ func (c *Client) handleSendPostMessage(jReq *jsonRequest) {
 		return
 	}
 
+	// We still want to return an error if for any reason the respone
+	// remains empty.
+	if httpResponse == nil {
+		jReq.responseChan <- &Response{
+			err: fmt.Errorf("invalid response"),
+		}
+	}
+
 	// Read the raw bytes and close the response.
 	respBytes, err := ioutil.ReadAll(httpResponse.Body)
 	httpResponse.Body.Close()