heap buffer overflow in libaom

Integer overflow in libaom internal function img_alloc_helper can lead to heap buffer overflow. This function can be reached via 3 callers:

* Calling aom_img_alloc() with a large value of the d_w, d_h, or align parameter may result in integer overflows in the calculations of buffer sizes and offsets and some fields of the returned aom_image_t struct may be invalid.
* Calling aom_img_wrap() with a large value of the d_w, d_h, or align parameter may result in integer overflows in the calculations of buffer sizes and offsets and some fields of the returned aom_image_t struct may be invalid.
* Calling aom_img_alloc_with_border() with a large value of the d_w, d_h, align, size_align, or border parameter may result in integer overflows in the calculations of buffer sizes and offsets and some fields of the returned aom_image_t struct may be invalid.