Correct bug in loopfilter initialization

The memset sets 16 bytes rather than the correct size of the
final array dimension (MAX_MODE_LF_DELTAS).

(In response to bug posted by Manjit Hota to codec-devel
and webm-discuss lists)

Change-Id: I8980f5aa71ddc9d7ef57c5b4700bc28ddf8651c7