fix: panic on zero-length inputs in Mux

[MozirDmitriy]

Description

Add explicit check in Mux to panic when inputs length is zero.
Prevents undefined behavior: previously n==0 led to bits.Len(n-1) on MaxUint and slicing inputs[:leftCount], causing non-obvious runtime panics and potential big.Int overflow in comparator bound construction.
Aligns with package style where invalid shape/size inputs trigger panic, while out-of-range selectors cause proof failure.

Type of change

• Bug fix (non-breaking change which fixes an issue)

How has this been tested?

go test -v ./std/selector
go test -v -run "Mux" ./std/selector
go test -v ./...

Checklist:

• I have performed a self-review of my code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• I have added tests that prove my fix is effective or that my feature works
• I did not modify files generated from templates
• golangci-lint does not output errors locally
• New and existing unit tests pass locally with my changes
• Any dependent changes have been merged and published in downstream modules