vp9_quantize_ssse3_x86_64: fix out of bounds write

eob is a pointer to a uint16_t. previously the code would store 64-bits
causing a crash or test failure with the right stack layout.

Change-Id: Ibd653baf323db114f2444951b9d8b00c596bf15a
diff --git a/vp9/encoder/x86/vp9_quantize_ssse3_x86_64.asm b/vp9/encoder/x86/vp9_quantize_ssse3_x86_64.asm
index 1d421f6..5703aa3 100644
--- a/vp9/encoder/x86/vp9_quantize_ssse3_x86_64.asm
+++ b/vp9/encoder/x86/vp9_quantize_ssse3_x86_64.asm
@@ -169,7 +169,7 @@
   pshuflw                         m7, m8, 0x1
   pmaxsw                          m8, m7
   pextrw                          r6, m8, 0
-  mov                           [r2], r6
+  mov                           [r2], r6w
   RET
 %endmacro