get_min_tx_size: assert() doesn't need an 'if'.
get_min_tx_size: assert() doesn't need an 'if'.

Change-Id: Id2be191fb48ed8d65b452499e1a1a1f470359321

GrokImageCompression/aom,GrokImageCompression/aom,GrokImageCompression/aom,GrokImageCompression/aom,GrokImageCompression/aom,GrokImageCompression/aom