giflib: 5.1.4 -> 5.2.1

5.1.5 says: Fix SF bug #113:
  Heap Buffer Overflow-2 in function DGifDecompressLine()
  in cgif.c.  This had been assigned CVE-2018-11490.
This *probably* also fixes a minor CVE-2019-15133 #68563 (DoS only).
(I couldn't find a good reference quickly.)

The changelogs are almost only bugfixes, and ABI tracker also looks OK:
https://abi-laboratory.pro/?view=timeline&l=giflib
so I gather this is suitable for direct push to nixpkgs staging-next.
I built several reverse dependencies locally.