sbclPackages.named-readtables: fix build with SBCL 2.6.0

SBCL commit 00eabf5 changed the readtable's base-char-macro-array to
store 0 instead of NIL for non-macro characters. Since (if 0 ...) is
truthy in Common Lisp, the named-readtables readtable iterator
incorrectly yields all characters, causing assertion failures in
check-reader-macro-conflict when packages like cl-interpol,
pythonic-string-reader, and antik-base merge readtables.

Apply upstream fix from melisgl/named-readtables@6eea566.

pkgs/development/lisp-modules/ql.nix

@@ -12,6 +12,15 @@ let
 
   overrides = (
     self: super: {
+      named-readtables = super.named-readtables.overrideLispAttrs (o: {
+        patches = (o.patches or [ ]) ++ [
+          (pkgs.fetchpatch {
+            name = "named-readtables-sbcl-fix.patch";
+            url = "https://github.com/melisgl/named-readtables/commit/6eea56674442b884a4fee6ede4c8aad63541aa5b.patch";
+            hash = "sha256-ZkmBz50tkJutCNhrgVTHyE+sxRjmL8y7YC7yewrmves=";
+          })
+        ];
+      });
       cl_plus_ssl = super.cl_plus_ssl.overrideLispAttrs (o: {
         nativeLibs = [ pkgs.openssl ];
       });