next() would read a truncated lead byte's "second byte" unconditionally,
even when that byte was actually the string's null terminator - leaving
index_ one past the terminator, so a subsequent end()/next() call read
one byte out of bounds. Now checks for the terminator before consuming
it, returning INVALID without advancing past it.
Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01L4QAoxV2KY7ek4PcZw3WvY
