Handle server name overflow properly

We need to make sure it is null terminated on truncation. We also
need to avoid giving a too large size argument or modern gcc will
complain.

vncviewer/vncviewer.cxx

       newServerName = loadViewerParameters(vncServerName);
       // This might be empty, but we still need to clear it so we
       // don't try to connect to the filename
-      strncpy(vncServerName, newServerName, VNCSERVERNAMELEN);
+      strncpy(vncServerName, newServerName, VNCSERVERNAMELEN-1);
+      vncServerName[VNCSERVERNAMELEN-1] = '\0';
     } catch (rfb::Exception& e) {
       vlog.error("%s", e.str());
       if (alertOnFatalError)
   try {
     const char* configServerName;
     configServerName = loadViewerParameters(NULL);
-    if (configServerName != NULL)
-      strncpy(defaultServerName, configServerName, VNCSERVERNAMELEN);
+    if (configServerName != NULL) {
+      strncpy(defaultServerName, configServerName, VNCSERVERNAMELEN-1);
+      defaultServerName[VNCSERVERNAMELEN-1] = '\0';
+    }
   } catch (rfb::Exception& e) {
     vlog.error("%s", e.str());
     if (alertOnFatalError)