Implement mitigation for GHSL-2021-045

This add a static inline version of `g_memdup2` if the version of glib we're
compiling against doesn't have the function.

GHSL-2021-045 was originally reported to glib at
https://gitlab.gnome.org/GNOME/glib/-/issues/2319.

More information about the entire situation can be found on the gnome
desktop-devel-list at
https://mail.gnome.org/archives/desktop-devel-list/2021-February/msg00000.html

Testing Done:
Compiled and ran unit tests.

Reviewed at https://reviews.imfreedom.org/r/482/