Fix WiFiUDP::peek return value when buffer is empty (#1796)

Author: igrr

libraries/ESP8266WiFi/src/WiFiUdp.cpp

@@ -236,7 +236,7 @@ int WiFiUDP::read(unsigned char* buffer, size_t len)
 int WiFiUDP::peek()
 {
     if (!_ctx)
-        return 0;
+        return -1;
 
     return _ctx->peek();
 }

libraries/ESP8266WiFi/src/include/UdpContext.h

@@ -190,7 +190,7 @@ class UdpContext
 
     int read()
     {
-        if (!_rx_buf || _rx_buf->len == _rx_buf_offset)
+        if (!_rx_buf || _rx_buf_offset == _rx_buf->len)
             return -1;
 
         char c = reinterpret_cast<char*>(_rx_buf->payload)[_rx_buf_offset];
@@ -215,8 +215,8 @@ class UdpContext
 
     char peek()
     {
-        if (!_rx_buf)
-            return 0;
+        if (!_rx_buf || _rx_buf_offset == _rx_buf->len)
+            return -1;
 
         return reinterpret_cast<char*>(_rx_buf->payload)[_rx_buf_offset];
     }