Gimp: gimp: heap buffer over-read in x cursor (xmc) thumbnail loader on crafted file

A flaw was found in GIMP’s X cursor (XMC) thumbnail loader. When GIMP generates a thumbnail for a crafted XMC file, it allocates a pixel buffer using a width * height size computed in 32-bit signed arithmetic. If that product overflows, the allocation is smaller than the true image extent. A subsequent GEGL buffer read uses the unwrapped dimensions and performs an out-of-bounds read on the heap, after integer overflow in the size calculation. This can crash GIMP or corrupt process memory.