* src/gig.cpp, src/gig.h: fixed behavior regarding the newly added support
for sample groups, the initial implementation assumed a group index of 0
in the .gig file to mean "no group assigned" and thus the possibility
that there might not be a group at all, but in fact the .gig format
assumes always at least one group (the default group), thus a group index
of 0 means "member of default group"