Use strnlen when checking for null byte

[edsrzf]

Buffers are not always null-terminated, so using strlen here could cause reads past the end of the buffer and cause Janet to incorrectly report that the string contains zeros because the lengths don't match.

There may be other instances to fix, but this fixes #1600.

[bakpakin]

Looks good. Most places where we convert buffers to strings, we should be calling janet_getcbytes or similar to ensure that final 0 byte of padding.