Heap buffer overflow in DXF polyline import

LibreOffice can import drawings in the DXF format used by CAD software. A heap buffer overflow existed when importing a DXF polyline. The point count taken from the file was truncated to a 16-bit value when the point buffer was sized, while the full count was used to fill it, so a polyline whose point count exceeded the 16-bit range was written past the end of the buffer. In fixed versions such oversized polylines are rejected.