Don't attempt to walk nonexistent directories

Fixes Homebrew/homebrew#11298.

Signed-off-by: Jack Nagel <jacknagel@gmail.com>

Library/Homebrew/cmd/doctor.rb

@@ -183,6 +183,7 @@ def check_for_broken_symlinks
   broken_symlinks = []
   %w[lib include sbin bin etc share].each do |d|
     d = HOMEBREW_PREFIX/d
+    next unless d.directory?
     d.find do |pn|
       broken_symlinks << pn if pn.symlink? and pn.readlink.expand_path.to_s =~ /^#{HOMEBREW_PREFIX}/ and not pn.exist?
     end

Library/Homebrew/cmd/prune.rb

@@ -10,6 +10,7 @@ module Homebrew extend self
     dirs = []
 
     %w[bin sbin etc lib include share Library/LinkedKegs].map{ |d| HOMEBREW_PREFIX+d }.each do |path|
+      next unless path.directory?
       path.find do |path|
         path.extend ObserverPathnameExtension
         if path.symlink?