Security: fix jas_stream_peek, guard zero-length unread loop to avoid size_t underflow

[xmoezzz]

When n is zero, the loop for (size_t i = n; i-- > 0;) never enters the body but still wraps i from 0 to SIZE_MAX, which triggers UBSan.
Add an early if (n == 0) return 0; to keep the behaviour unchanged while avoiding the warning.

Fix #407