Fix hangs and segfaults when egregiously long symbol is used

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.
It would be better to use `nb > SYM_POOL_SIZE`.