safety fix for glob's vla usage: disallow patterns longer than PATH_MAX

this actually inadvertently disallows some valid patterns with
redundant / or * characters, but it's better than allowing unbounded
vla allocation.

eventually i'll write code to move the pattern to the stack and
eliminate redundancy to ensure that it fits in PATH_MAX at the
beginning of glob. this would also allow it to be modified in place
for passing to fnmatch rather than copied at each level of recursion.