mtr: import a patch from upstream to fix segfault

There is a buffer overflow problem in mtr which is fixed upstream, and
can be reproduced with:

❯ mtr -z -4 -c 5 -w example.com
Start: 2023-09-12T09:38:31+0000
*** buffer overflow detected ***: terminated
[1] 293109 abort (core dumped) mtr -z -4 -c 5 -w example.com

pkgs/tools/networking/mtr/default.nix

···
 { stdenv
 , lib
 , fetchFromGitHub
+, fetchpatch
 , autoreconfHook
 , pkg-config
 , libcap
···
     rev = "v${version}";
     sha256 = "sha256-f5bL3IdXibIc1xXCuZHwcEV5vhypRE2mLsS3A8HW2QM=";
   };
+
+  patches = [ (fetchpatch { # https://github.com/traviscross/mtr/pull/468
+                url = "https://github.com/traviscross/mtr/commit/5908af4c19188cb17b62f23368b6ef462831a0cb.patch";
+                hash = "sha256-rTydtU8+Wc4nGEKh1GOkhcpgME4hwsACy82gKPaIe64=";
+              }) ];
 
   # we need this before autoreconfHook does its thing
   postPatch = ''