wildtest: don't read past the buffer when scanning a test line

wildtest: don't read past the buffer when scanning a test line

main()'s line parser stepped through the fgets() buffer with `*++s` in
three places without first checking for the terminating NUL, so a test
line whose last token runs to the end of the buffer (e.g. a final line
with no trailing newline) could advance s past the NUL and read out of
bounds.

Guard the flag-separator check and rewrite the two whitespace-skip loops
so they never step past the NUL. No behaviour change for well-formed
input: the existing wildtest.txt still passes, and the crafted overflow
input is now clean under valgrind.

Fixes #776
Reported-by: vikk777 (@vikk777)