Encoder segfaults in global motion setup when frame resize is enabled at speed >= 6

[xinzhao-apple]

What version / commit were you testing with?

v1.0.0-101-g885b0a892

What steps will reproduce the problem?

1. Encode at --cpu-used=8 in 1-pass CBR with global motion on and a resize that gives the
   key frame and the inter frames different coded sizes:

   ./avmenc --cpu-used=8 --passes=1 --threads=1 \
            --width=256 --height=192 --yv12 --input-bit-depth=8 --bit-depth=8 \
            --resize-mode=1 --resize-denominator=9 --resize-kf-denominator=11 \
            --enable-global-motion=1 --end-usage=cbr \
            --limit=20 --ivf -o out.ivf noise_256x192.yuv

Expected behavior

The encoder should encode the clip, or report a diagnosable error.

Actual behavior

avmenc segfaults (exit 139 / SIGSEGV) on the first inter frame, immediately after the
key frame is coded at a different resolution. Fully deterministic:

POC: 0 [ KEY ][Level:1][Q:  8]:  45358 Bytes  186x140
POC: 0 [INTER][Level:2][Q:177]:   2613 Bytes  228x171
Segmentation fault