Simple changes to aom_decode_frame_from_obus.
Simple changes to aom_decode_frame_from_obus.

Document the return value of aom_decode_frame_from_obus. Validate
payload_size early and correctly, and remove redundant checks. Remove a
redundant av1_init_read_bit_buffer call.

BUG=aomedia:1932

Change-Id: I450bf59a013ee19d36c6beb593cda0b92778adfc

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