libsass: add patch for CVE-2018-19827 (PR #61673)

vcunat switched to upstream commit that's reachable from master.

(cherry picked from commit f9f3d6b210f0ca946ed202d3b7f89ec78988c130)

pkgs/development/libraries/libsass/default.nix

···
     sha256 = "0w6v1xa00jvfyk4b29ir7dfkhiq72anz015gg580bi7x3n7saz28";
   };
 
+  patches = [
+    (fetchpatch {
+      name = "CVE-2018-19827.patch";
+      url = "https://github.com/sass/libsass/commit/b21fb9f84096d9927780b86fa90629a096af358d.patch";
+      sha256 = "0ix12x9plmpgs3xda2fjdcykca687h16qfwqr57i5qphjr9vp33l";
+    })
+  ];
+
   preConfigure = ''
     export LIBSASS_VERSION=${version}
   '';