Only look for libraries in lib if it exists

Fixes Homebrew/homebrew#40630.

Library/Homebrew/keg_fix_install_names.rb

@@ -148,7 +148,7 @@ class Keg
   end
 
   def find_dylib name
-    lib.find { |pn| break pn if pn.basename == name }
+    lib.find { |pn| break pn if pn.basename == name } if lib.directory?
   end
 
   def mach_o_files