pat: treat {0,} quantifier as * to avoid nil-pointer panic

[SAY-5]

Fixes #558. The isMinimumOnly branch in faFromQuantifiedAtom builds a chain of mandatory steps and then loops the last one back, but for {0,} the loop runs zero times, so lastState stays nil and the append on it panics. {0,} has no mandatory steps and means the same thing as *, so I handle that case the same way the * branch does.

Added an x{0,}y entry to regexpSamples (matches y/xy/xxxy, rejects x/z/xx); without the fix it panics during NFA construction.

[timbray]

Thanks! First glance looks good. Heading into a busy weekend, may not get to a closer look for a couple days.

[timbray]

Actually I realized how short it is, lgtm, thanks.

In future please sign your PRs, not with commit -s but by setting up a keypair with GitHub, years since I did it so I can't remember how but pretty straightforward I think.