Midi is corrupted error when char is unsigned

[felipemanga]

The following line (MidiTrack.cpp, line 236) assumes char is signed:
keySig = static_cast<char>(readByte()); // force sign conversion ...
This fails on platforms where it isn't.
A potential fix is:
keySig = static_cast<signed char>(readByte());

[Martchus]

I suggest you create a PR. Since this project isn't maintained very actively it may take a while until it gets merged, though.