Fix eobs buffer overflow caused by SSSE3 quantize_fp

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

av1/encoder/x86/quantize_ssse3_x86_64.asm

@@ -174,7 +174,7 @@ cglobal quantize_%1, 0, %2, 15, coeff, ncoeff, skip, zbin, round, quant, \
   pshuflw                         m7, m8, 0x1
   pmaxsw                          m8, m7
   pextrw                          r6, m8, 0
-  mov                           [r2], r6
+  mov                           [r2], r6w
   RET
 
   ; skip-block, i.e. just write all zeroes