Avoid overflow in calc_iframe_target_size am: 5480c1d281 am: 4b95222aaa

Original change: https://googleplex-android-review.googlesource.com/c/platform/external/libvpx/+/15148150

Change-Id: Ib8e84b2f6f12190617bc9f2b707f472c79a6b41c