Encoder fails when --enable-rect-partitions=0

[xinzhao-apple]

What version / commit were you testing with?

v1.0.0-90-g4bdc67918

What steps will reproduce the problem?

1. Build avmenc normally (release, no special configure flags).

2. Encode any clip with --enable-rect-partitions=0, --global-error-resilient=1
   and --enable-warp-delta=0, using at least 2 frames so that an inter frame is
   coded:

   avmenc --width=480 --height=270 --limit=3 --cpu-used=5 --threads=8 \
          --enable-rect-partitions=0 \
          --global-error-resilient=1 \
          --enable-warp-delta=0 \
          -o out.obu ParkJoy_480x270.y4m
   

3. The encoder fails on the first inter frame (POC 1).

Reproduced on several unrelated inputs and resolutions, such as 480x270 (ParkJoy),
1920x1080 (Slides2r).

Expected behavior

The encode completes and produces a decodable bitstream.

Actual behavior

POC:     0 [ KEY ][Level:1][Q:160]:       2931 Bytes, 1081.1ms  480x270
Stream 0: Failed to encode frame: Unspecified internal error
    The same superblock is recoded twice. Infinite loop detected?