vp9.read_inter_block_mode_info: return on corruption

vp9.read_inter_block_mode_info: return on corruption

with block sizes < 8x8 previously only the inner loop was aborted. this
could cause propagation of invalid motion vectors to scale_mv().

this quiets integer sanitizer warnings of the form:
vp9/common/vp9_mvref_common.h:239:18: runtime error: implicit conversion
from type 'int' of value 32768 (32-bit, signed) to type 'int16_t' (aka
'short') changed the value to -32768 (16-bit, signed)

Bug: b/229626362
Change-Id: I58b5a425adf21542cbf4cc4dd5ab3cc5ed008264