output/srzip: fix potential "use after free"

output/srzip: fix potential "use after free"

The compiler marks a potential use after free, which the current
implementation won't trigger. The error only occurs when a sigrok
channel is neither logic nor analog.

Address the issue nevertheless, to silence the compiler warning, and to
protect against possible programming errors when a future implementation
should support more channel types.

This was reported by clang's scan-build.