fix: add C >= 128 guards + fallback to all UTF-8 continuation validators in cow_ws (crash on malformed text frames)

[dl-alexandre]

Summary

WebSocket text/close frames must be valid UTF-8. The state machine in cow_ws:validate_text/2 (and validate_sN helpers) assumed continuation states (s2-s8) would only ever see bytes >= 128.

A low byte (< 128) while in one of those states (e.g. truncated/malformed frame from network or adversarial input) caused element(C-127, utf8_class()) to receive an out-of-range index and crash (badarg; after partial guard also function_clause).

This is the root cause of the crash reported in #158.

Fix (smallest defensible change)

• Add when C >= 128 guard to the main clause of validate_s2 .. validate_s8 (consistent with validate_s0).
• Add a fallback clause for any other non-empty input in those states: validate_sN(<<,/bits>>) -> 1; (return the 'bad encoding' sentinel).
• The <<>> terminal clauses are unchanged (return the state number for 'more').

Valid UTF-8 paths (ASCII fast path, 2/3/4-byte sequences, fragmented text frames) are unaffected.

Testing

Local verification (no full erlang.mk TEST_DEPS fetch required):

• erlc compiles the module cleanly (no warnings after anon-var cleanup).
• Exercised the paths via the exported parse_payload/9 (which drives validate_payload -> validate_text for text frames):
  • ASCII and valid multi-byte (e.g. cent 0xC2A2) -> {ok, Payload, 0, <<>>}
  • Partial first-byte + second -> more then {ok}
  • Low ASCII byte while Utf8State=2 (s2) -> {error, badencoding} with zero crash.

This makes the UTF-8 validator robust against the reported class of input.

Fixes #158

---

(Contrib loop: claimed via work claim, investigated, smallest targeted change + manual property-like smoke on the exact crash scenario, self-verified compile+exec, now gating.)

[essen]

Thanks but #159 already has the fix.