network_server example: Detect count=0 in read_callback() (#421)

Previously the example code would falsely detect EOF condition if
called with count = 0.

examples/network_server/common.c

@@ -19,6 +19,9 @@ static bool read_callback(pb_istream_t *stream, uint8_t *buf, size_t count)
     int fd = (intptr_t)stream->state;
     int result;
     
+    if (count == 0)
+        return true;
+
     result = recv(fd, buf, count, MSG_WAITALL);
     
     if (result == 0)