strv: add an additional overflow check when enlarging strv()s

https://bugs.freedesktop.org/show_bug.cgi?id=76745

This also adds:
strv: use realloc_multiply() to check for multiplication overflow
by Michal Schmidt <mschmidt@redhat.com>

This could overflow on 32bit, where size_t is the same as unsigned.

Signed-off-by: Anthony G. Basile <blueness@gentoo.org>