Heap buffer overflow in EMF+ gradient brush import

LibreOffice can import EMF+ graphics, which may be embedded in documents. A heap buffer overflow existed when importing an EMF+ gradient brush. The number of gradient blend points was read from the file and used to compute an allocation size, but that multiplication could overflow, so a small buffer was allocated and then filled as if it were large, writing past its end. In fixed versions the blend-point count is checked against the data actually available before allocating.