ffmpeg: Crash in gmc_mmx

Project:
https://git.ffmpeg.org/ffmpeg.git

Detailed report: https://oss-fuzz.com/testcase?key=4608518562775040

Project: ffmpeg
Fuzzer: libFuzzer_ffmpeg_AV_CODEC_ID_MPEG4_fuzzer
Job Type: libfuzzer_asan_ffmpeg
Crash Type: UNKNOWN READ
Crash Address: 0x620ffffffbf8
Crash State:
gmc_mmx
gmc_motion
ff_mpv_reconstruct_mb

Sanitizer: address (ASAN)

Recommended Security Severity: Medium

Regressed: https://oss-fuzz.com/revisions?job=libfuzzer_asan_ffmpeg&range=201704301625:201705042156

Reproducer Testcase: https://oss-fuzz.com/download?testcase_id=4608518562775040

Issue filed automatically.

See https://github.com/google/oss-fuzz/blob/master/docs/reproducing.md for more information.

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