stdenv/problems: avoid a lookup for meta.broken

The stats may be lying to me when they say that this saves 1.86% of
attrset lookups, and `?` may just not be accurately tracked. But at
worst, performance will stay the same, since the check will fail for all
non-broken packages. And who knows, maybe it does help, by nature of not
checking the value!

pkgs/stdenv/generic/problems.nix

@@ -128,9 +128,9 @@ rec {
         if allowBroken then
           attrs: false
         else if config ? allowBrokenPredicate then
-          attrs: attrs.meta.broken or false && !allowBrokenPredicate attrs
+          attrs: attrs ? meta.broken && attrs.meta.broken && !allowBrokenPredicate attrs
         else
-          attrs: attrs.meta.broken or false;
+          attrs: attrs ? meta.broken && attrs.meta.broken;
       value.message = "This package is broken.";
     }
   ];