The commit fixes the segfault shown in  test AV1/ActiveMapTestLarge
when build with the following config:
../../aom/configure --disable-install-docs --enable-unit-tests
--enable-debug --disable-optimizations --enable-aom-highbitdepth
--enable-experimental --enable-ans --enable-bidir-pred
--enable-dual-filter --enable-entropy --enable-ext-inter
--enable-ext-interp --enable-ext-intra --enable-ext-partition
--enable-ext-partition-types --enable-ext-refs --enable-ext-tx
--enable-loop-restoration --enable-filter-intra --enable-motion-var
--enable-new-quant --enable-ref-mv --enable-supertx --enable-var-tx

BUG=aomedia:99

Change-Id: I900d5e1d5458c482a1f0a3dde769c1ad93acdca7