bugfix: use oxcf width/height for reinit check

use oxcf instead of common in check to Reinit the
lookahead buffer if the frame size changes
prior behavior would cause assertion fail/crash

first observed in:
support changing resolution with vpx_codec_enc_config_set

Change-Id: Ib669916ca9b4f206d4cc3caab5107e49d39a36aa