The SSSE3 version of av1_quantize_fp stored the new value of eob
with a 32bit mov instruction, but the eob values are stored in an
array of uint16_t. This caused the last eob store to write 2 extra
bytes off the end of the eob array.

This doesn't seem to currently be a problem on master, but it
causes errors for my RDO refactoring under certain conditions.

Change-Id: I9f7e9c4e40b37b8a2c232b87d170d24593c1066c