Fix Valgrind warning in av1_pick_filter_restoration

Some array elements were defined and left uninitialised. This wasn't causing a
problem, as the elements were later ignored, but it did cause Valgrind to
produce warnings.

The function now initialises the full array immediately after its definition in
order to quiet these warnings.

BUG=aomedia:1244

Change-Id: I5083f1f4008cb3ab70a4af4d1d2573dee8793303