make globfree safe after failed glob from over-length argument

make globfree safe after failed glob from over-length argument

commit 0dc99ac413d8bc054a2e95578475c7122455eee8 added input length
checking to avoid unsafe VLA allocation, but put it in the wrong
place, before the glob_t structure was zeroed out. while POSIX isn't
clear on whether it's permitted to call globfree after glob failed
with GLOB_NOSPACE, making it safe is clearly better than letting
uninitialized pointers get passed to free in non-conforming callers.

while we're fixing this, change strlen check to the idiomatic strnlen
version to avoid unbounded input scanning before returning an error.