Assert a vpx_img_set_rect call always succeed

The vpx_img_set_rect() call at the end of img_alloc_helper() always
succeeds, so assert its return value is equal to 0.

A port of the changes to aom/src/aom_image.c in the libaom CLs
https://aomedia-review.googlesource.com/c/aom/+/90307 and
https://aomedia-review.googlesource.com/c/aom/+/190011.

Bug: webm:1850
Change-Id: I559820db245a596b4aed2042bfa7ebe7dd2d69b7
1 file changed