engine: fix some signed char cases

engine/common/net_buffer.c

@@ -386,7 +386,7 @@ qboolean MSG_WriteString( sizebuf_t *sb, const char *pStr )
 	{
 		do
 		{
-			MSG_WriteChar( sb, *pStr );
+			MSG_WriteChar( sb, (signed char)*pStr );
 			pStr++;
 		} while( *( pStr - 1 ));
 	}

engine/server/sv_game.c

@@ -2729,7 +2729,7 @@ pfnWriteChar
 */
 void GAME_EXPORT pfnWriteChar( int iValue )
 {
-	MSG_WriteChar( &sv.multicast, (char)iValue );
+	MSG_WriteChar( &sv.multicast, (signed char)iValue );
 	svgame.msg_realsize++;
 }