fix: handle match_mask == 0 in NEON ctzll to avoid undefined behavior

[lalinsky]

When all 16 bytes match the allowed range, match_mask becomes 0 after the bitwise NOT. Calling __builtin_ctzll(0) is undefined behavior.

The code expects match_len == 16 when all bytes match (so the branch is skipped and p += 16 continues the loop), but this relied on ctzll(0) returning 64, which is not guaranteed.

Example panic on macOS ARM64:

thread 44856 panic: passing zero to ctz(), which is not a valid argument
src/llhttp/llhttp.c:2654:21: in llhttp__internal__run
        match_len = __builtin_ctzll(match_mask) >> 2;
                    ^

Fix by explicitly checking for match_mask == 0 and setting match_len = 16.

Related: lalinsky/dusty#14