Make sure clipboard uses \n line endings

This is required by the protocol so we should make sure it is
enforced. We are tolerant of clients that violate this though and
convert incoming clipboard data.
diff --git a/common/rfb/SMsgWriter.cxx b/common/rfb/SMsgWriter.cxx
index 6a2c2ba..f0748ff 100644
--- a/common/rfb/SMsgWriter.cxx
+++ b/common/rfb/SMsgWriter.cxx
@@ -80,6 +80,9 @@
 
 void SMsgWriter::writeServerCutText(const char* str, int len)
 {
+  if (memchr(str, '\r', len) != NULL)
+    throw Exception("Invalid carriage return in clipboard data");
+
   startMsg(msgTypeServerCutText);
   os->pad(3);
   os->writeU32(len);