Bug in decode variable int function breaks MIDI file parsing

Bug in decode variable int function breaks MIDI file parsing: a task in harbor-swesmith-rl-artifacts: Harbor SWE-Smith 强化学习数据产物 (Harbor dataset). I found a bug in the decode variable int function in mido/midifiles/meta.py that causes issues when parsing certain MIDI files.

The task

I found a bug in the `decode_variable_int` function in `mido/midifiles/meta.py` that causes issues when parsing certain MIDI files.

Part of keryszhan/harbor-swesmith-rl-artifacts.