Coverity shows a lot of warnings for a2m-v2.cpp (possible overruns etc), will try to handle them here.

What's fixed:

• Resource leak Coverity warning (memory leak when reading corrupt tune)
• Missing break in switch Coverity warning (low chance of executing though)
• Out-of-bounds Coverity warning; added more range checks in set_ins_volume()
• Always true branch Coverity warning: fixed macro vibrato for the instruments, most prominent in ab_julia.a2t and fm63b_rv.a2m tunes; also updated refs for tests
• Now properly initialize some buffers before playing with (-1) value, this fixes &2?, &3?, ZC? and ZD? effects (notecut, note delay and pattern loop)