Make sure server name is always a valid string

Otherwise we can get crashes on NULL dereference. This should only
happen on reverse connections where we don't have a server address.

common/rfb/CConnection.cxx

@@ -52,7 +52,7 @@ CConnection::CConnection()
     supportsDesktopResize(false), supportsLEDState(false),
     is(0), os(0), reader_(0), writer_(0),
     shared(false),
-    state_(RFBSTATE_UNINITIALISED),
+    state_(RFBSTATE_UNINITIALISED), serverName(strDup("")),
     pendingPFChange(false), preferredEncoding(encodingTight),
     compressLevel(2), qualityLevel(-1),
     formatChange(false), encodingChange(false),
@@ -68,6 +68,13 @@ CConnection::~CConnection()
   close();
 }
 
+void CConnection::setServerName(const char* name_)
+{
+  if (name_ == NULL)
+    name_ = "";
+  serverName.replaceBuf(strDup(name_));
+}
+
 void CConnection::setStreams(rdr::InStream* is_, rdr::OutStream* os_)
 {
   is = is_;

common/rfb/CConnection.h

@@ -48,7 +48,7 @@ namespace rfb {
     // which we are connected.  This might be the result of getPeerEndpoint on
     // a TcpSocket, for example, or a host specified by DNS name & port.
     // The serverName is used when verifying the Identity of a host (see RA2).
-    void setServerName(const char* name_) { serverName.replaceBuf(strDup(name_)); }
+    void setServerName(const char* name_);
 
     // setStreams() sets the streams to be used for the connection.  These must
     // be set before initialiseProtocol() and processMsg() are called.  The