diagnostic: handle HOMEBREW_TEMP mktmpdir failure.

Fixes #1223.

Library/Homebrew/diagnostic.rb

@@ -644,6 +644,7 @@ module Homebrew
         real_cellar = HOMEBREW_CELLAR.realpath
         where_cellar = volumes.which real_cellar
 
+        begin
           tmp = Pathname.new(Dir.mktmpdir("doctor", HOMEBREW_TEMP))
           begin
             real_tmp = tmp.realpath.parent
@@ -651,6 +652,10 @@ module Homebrew
           ensure
             Dir.delete tmp
           end
+        rescue
+          return
+        end
+
         return if where_cellar == where_tmp
 
         <<-EOS.undent