fix b1c6nbt model invalid mid_num_channels setting.

[b05902062]

Issue

When running b1c6nbt with c++ engine, it will trigger this error.
terminate called after throwing an instance of 'StringError' what(): Error loading or parsing model file /b1c6nbt-s31704192-d83750556/model.bin.gz: trunk: all numbers of channels must be positive

trunk exported here

def write_trunk(name,model):
writeln("trunk")
writeln(len(model.blocks))
writeln(model.c_trunk)
writeln(model.c_mid)
writeln(model.c_mid-model.c_gpool)
writeln(model.c_gpool)
writeln(model.c_gpool)

Error thrown here

TrunkDesc::TrunkDesc(istream& in, int vrsn, bool binaryFloats) {
in >> name;
version = vrsn;
in >> numBlocks;
in >> trunkNumChannels;
in >> midNumChannels;
in >> regularNumChannels;
int dilatedNumChannels; //unused
in >> dilatedNumChannels;
in >> gpoolNumChannels;

if(in.fail())
throw StringError(name + ": trunk failed to parse num blocks or various channel parameters");
if(numBlocks < 1)
throw StringError(name + ": trunk num blocks must be positive");
if(
trunkNumChannels <= 0 || midNumChannels <= 0 || regularNumChannels <= 0 ||
gpoolNumChannels <= 0)
throw StringError(name + ": all numbers of channels must be positive");

[b05902062]

Probably a typo. 6 is more consistent with others' config.