[Bug c/72783] Fortify scanf %s, %[ conversion specifiers

[msebor at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=72783

--- Comment #3 from Martin Sebor <msebor at gcc dot gnu.org> ---
I've run out of time to implement this in GCC 7 but I'll see about handling it
in GCC 8.