faad2 Merge Request #1: Fix possible out-of-bounds (merged)

Depending on build options and stream contents either one or two channels will be used; this change proactively checks that 2 channel slots are available.
Existing code does everything else correctly (i.e. advances slot position by the number of output channels actually used).

[Fabian Greffrath]

Thanks for your contribution! Please follow the new upstream GIT repository where I have already applied your fix:
https://github.com/knik0/faad2