Fix GP3 writeOldChord/readOldChord asymmetry

[kaizenman]

Bug

GP3File.writeOldChord unconditionally writes the 6 fret ints after firstFret, but GP3File.readOldChord only reads them when firstFret != 0:

# writeOldChord (current)
self.writeIntByteSizeString(chord.name)
self.writeI32(chord.firstFret)
for fret in clamp(chord.strings, 6, fillvalue=-1):
    self.writeI32(fret)              # always 6 * i32

# readOldChord
chord.name = self.readIntByteSizeString()
chord.firstFret = self.readI32()
if chord.firstFret:                  # only when firstFret != 0
    for i in range(6):
        fret = self.readI32()
        ...

When a chord with firstFret == 0 is written and parsed back, 24 unread bytes remain in the stream. Every subsequent read is misaligned; a later note's bend type byte ends up being read as -1, raising:

ValueError: -1 is not a valid BendType

Reproduction

Any GP3 file whose chord has firstFret == 0 (i.e. newFormat = False and no trailing fret list) triggers the crash on a library-only roundtrip:

import guitarpro as gp, io
song = gp.parse("file_with_old_format_chord.gp")
buf = io.BytesIO()
gp.write(song, buf, version=song.versionTuple)
gp.parse(io.BytesIO(buf.getvalue()))  # GPException: -1 is not a valid BendType

The included fixture tests/Chord Old Format.gp3 (a 1-track minimal variant of the existing Chords.gp3 with the chord's newFormat set to False and firstFret set to 0) reproduces this deterministically.

Fix

Write the fret list only when firstFret is non-zero, mirroring the read logic and the byte layout emitted by Guitar Pro itself:

def writeOldChord(self, chord):
    self.writeIntByteSizeString(chord.name)
    self.writeI32(chord.firstFret)
    if chord.firstFret:
        for fret in clamp(chord.strings, 6, fillvalue=-1):
            self.writeI32(fret)

Verification

• With the new fixture added to TESTS in test_conversion.py, the existing testReadWriteEquals fails on the current master with the exact error above and passes after the fix.
• All 191 existing tests continue to pass.
• Cross-checked against TuxGuitar — files produced by real Guitar Pro software with firstFret == 0 load correctly in both TuxGuitar and PyGuitarPro, whereas files produced by the buggy writeOldChord fail to load in both. This confirms the read side (and the fixed write) matches the actual Guitar Pro byte layout.

Scope

writeOldChord is only invoked for GP3 (.gp) files with chord.newFormat == False. GP4/GP5 use writeNewChord, which is symmetric and unaffected.

[Perlence]

@kaizenman Cheers!