The SSE4 function filter_intra_edge_sse4_1() reads data slightly
past the initialized part of the array. Those data are discarded
later, but causes a valgrind warning. This change avoids the warning
by initializing the array an extra +16 positions.

BUG=aomedia:868

Change-Id: Ib610492cff91492ae379c5d62895773f8747c4bc