fix(tests/video_cmp): initialize grab_failure_cnt before the compare loop

[Anai-Guo]

Problem

compare() in the ffmpeg integration test helper (video_cmp.py) counts consecutive failed frame grabs to bail out of a potential endless loop:

for _ in range(frame_count):
    retval1 = video1.grab()
    retval2 = video2.grab()

    if not retval1 or not retval2:
        grab_failure_cnt += 1          # <-- never initialized
        if grab_failure_cnt >= 10:
            raise Exception("Grab failed too much ...")
    else:
        grab_failure_cnt = 0

grab_failure_cnt is only ever assigned in the else branch (reset to 0 after a successful grab). The very first time either video1.grab() or video2.grab() returns False, grab_failure_cnt += 1 executes before any binding exists, so Python raises UnboundLocalError: local variable 'grab_failure_cnt' referenced before assignment — masking the real "grab failed" condition the counter was meant to detect.

Fix

Initialize grab_failure_cnt = 0 alongside similar before the loop, so the failure path counts as designed and the endless-loop guard can trigger.

Both identical copies (ffmpeg_basic/ and ffmpeg_bypass/) are fixed.

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