Backport CVE-2021-27218 integer overflow fix to GLib 2.58

Debian 10 contains GLib 2.58 and is supported for a bit more than 1 more year, so I need to backport the CVE-2021-27218 integer overflow fix to that version. Similar to !2000 (merged), I'd like to do this upstream.

I do not intend this to imply any particular upstream support for GLib 2.58, and in particular I don't plan to make any new GLib 2.58.x releases.

Commits

• gstrfuncs: Add internal g_memdup2() function

  From: @pwithnall

  (Same as the first commit in !2000 (merged). It's a straightforward cherry-pick from 2.66.)

• gbytearray: Do not accept too large byte arrays

  From: @krnowak

  (Straightforward cherry-pick from 2.66.)