[LOW] Prevent Unpacker#skip_nil from replaying nil indefinitely

[OskarEichler]

Summary

CBOR::Unpacker#skip_nil consumes the nil byte from the input buffer but leaves it cached as the current head byte. Every later skip_nil call therefore returns true for the same value, and the next read also replays nil instead of returning the following object.

This resets the cached head after a successful skip so the stream advances exactly once.

Reproduction

require "cbor"

unpacker = CBOR::Unpacker.new
unpacker.feed("\xf6\x01".b)

p 3.times.map { unpacker.skip_nil }
p unpacker.read

0.5.10.3 prints [true, true, true] and then nil. A caller using while unpacker.skip_nil never reaches the following object. This branch prints [true, false, false] and then 1.

Verification

• Exact 0.5.10.3 release and patched behavior were compared with the focused model above.
• A non-nil head remains cached for the following read, preserving the intended peek behavior.
• Empty input still raises EOFError.
• Existing suite: 170 examples, 0 failures on Ruby 4.0.6.
• rake compile, gem build cbor.gemspec, and git diff --check pass.

No tests were changed because this repository asks contributors not to add or modify tests.

Scope and limitations

The affected entry point is the low-level streaming Unpacker#skip_nil API. The infinite loop requires a caller to repeat skip_nil; it remains interruptible Ruby code, unlike a native-extension loop. Normal CBOR.decode does not use this method.

Breaking changes

skip_nil now consumes a nil exactly once, as its name and documentation imply. Code that relied on repeatedly observing the same skipped nil will behave differently; valid stream decoding is otherwise unchanged.