libvpx

vp9_loop_filter_frame_mt: defer allocations

the code currently checks whether the allocation has been done instead
of allocating on the first frame.
since:
4f27202 vp9: fix crash in mt loopfilter w/corrupt file

this change defers the allocation until the loop filter is used.

Change-Id: I660c1b7f34e713a8dd9884483f01d23b9847366e