Fix ASan bug in masked variance unit test
Fix ASan bug in masked variance unit test

The masked variance functions can read slightly off the
(nominal) end of their input array. This is not a problem,
since the extra data is never used, and when the functions
are used in the encoder proper, they will just read data
from the frame border.

However, the arrays in the test case had not been properly
padded, causing ASan to complain about reading off the end
of an array.

Also remove an accidental comment in the code.

BUG=aomedia:569

Change-Id: Icb5cf4d39ee50fc6e1b807f483940d7a7b12ce63

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