fix nv12 offset

Fix nv12 64 bit gcc increment.
Should be 16 bytes, but was 0x16 causing memory corruption.

TBR=harryjin@google.com
BUG=libyuv:492
Committed: https://chromium.googlesource.com/libyuv/libyuv/+/8fb2048e9fdb383503be75597f07658a0ca479e2

[fbarchard, 2015-09-24 16:49:34]

fbarchard@chromium.org changed reviewers:
+ fbarchard@chromium.org

[fbarchard, 2015-09-24 16:49:34]

c:\intelsde\sde -ast -hsw -- out\Release_x64\libyuv_unittest.exe
--gtest_catch_exceptions=0 --gtest_filter=*NV12ToARGB*
Note: Google Test filter = *NV12ToARGB*
[==========] Running 4 tests from 1 test case.
[----------] Global test environment set-up.
[----------] 4 tests from libyuvTest
[ RUN      ] libyuvTest.NV12ToARGB_Any
[       OK ] libyuvTest.NV12ToARGB_Any (199 ms)
[ RUN      ] libyuvTest.NV12ToARGB_Unaligned
[       OK ] libyuvTest.NV12ToARGB_Unaligned (112 ms)
[ RUN      ] libyuvTest.NV12ToARGB_Invert
[       OK ] libyuvTest.NV12ToARGB_Invert (87 ms)
[ RUN      ] libyuvTest.NV12ToARGB_Opt
[       OK ] libyuvTest.NV12ToARGB_Opt (85 ms)
[----------] 4 tests from libyuvTest (546 ms total)

[----------] Global test environment tear-down
[==========] 4 tests from 1 test case ran. (776 ms total)
[  PASSED  ] 4 tests.

[fbarchard1, 2015-09-24 17:19:22]

Committed patchset #2 (id:20001) manually as
8fb2048e9fdb383503be75597f07658a0ca479e2 (presubmit successful).