Fix hangs and segfaults when egregiously long symbol is used

[danluu]

Julia hangs when I have a program with a symbol of length 2**19. It then segfaults when I ^C. Longer symbols segfault immediately. Oddly, a symbol of length 2**19 causes julia to segfault immediately when I run it under gdb.

This causes an julia to error out instead hanging or segfaulting.

[JeffBezanson]

It would be better to use nb > SYM_POOL_SIZE.

[jiahao]

Dare I ask why you have symbols of length >=524,288?

[StefanKarpinski]

Jeez, that's a big symbol.

[danluu]

Ok. Used nb instead of length.

@jiahao , would you believe any of the following?

1. They say a picture is worth a thousand words, so I wrote an emacs mode that lets me use PNGs as variable names. Turns out, pictures are often longer than a thousand words.
2. I translated a java program to julia without renaming any of the variables.
3. I have a philosophical objection to the style guide's inistence on squishcase + short variable names, so I name my variables by including all words except the ones I mean.
4. You gotta name your variables something, y'know?

[quinnj]

💯

[kmsquire]

That's a very worthy @jiahao response! :-)

[ivarne]

This looks like a 0.3 backport candidate. Will cherry-pick as soon as it is merged 🍒

[JeffBezanson]

> would work instead of >=, but after that many characters who's really keeping track :)

[ivarne]

Backported to release-0.3 in 8c1b105