Avoid overflow in calc_iframe_target_size am: 5480c1d281 am: 4b95222aaa am: 05928920ff

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

Change-Id: Ic5ed5179e4008283a64ee8995728f59c8a781d4e