decoder: internal 'always false' term, handle invalid skip and channel

Introduce an "always false" type for .wait() terms. Map invalid counts
of skip conditions (zero or negative numbers) as well as invalid channel
references for level/edge conditions to this type which never matches.
Keep this "always false" term type an internal detail of the common
support code.

This is most robust and least intrusive at the same time, it keeps the
existing API, and simplifies the implementation of Python decoders for
rare edge cases (optional input signals or optional features, handling
of initial samples at the very start of a capture).

This commit passes sample counts internally in a signed data type. This
is essential for proper operation, and the loss of one bit out of 64
shall not be considered a severe limitation.

This fixes bug #1444.

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