Merge some file writing extension checks from OpenJK.

Thanks Ensiform.
https://github.com/JACoders/OpenJK/commit/05928a57f9e4aae15a3bd0
https://github.com/JACoders/OpenJK/commit/ef124fd0fc48af164581176

code/qcommon/common.c

@@ -2975,6 +2975,13 @@ void Com_WriteConfig_f( void ) {
 		return;
 	}
 
+
+	if (!COM_CompareExtension(filename, ".cfg"))
+	{
+		Com_Printf("Com_WriteConfig_f: Only the \".cfg\" extension is supported by this command!\n");
+		return;
+	}
+
 	Q_strncpyz( filename, Cmd_Argv(1), sizeof( filename ) );
 	COM_DefaultExtension( filename, sizeof( filename ), ".cfg" );
 	Com_Printf( "Writing %s.\n", filename );