Formula#system: don't try to close a closed stream

The new formula debugger users continuations to jump back up the stack,
and thus anything that has the potential to be executed twice (i.e.
ensure blocks) needs to to be essentially idempotent.

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

Library/Homebrew/formula.rb

@@ -552,7 +552,7 @@ protected
   rescue ErrorDuringExecution => e
     raise BuildError.new(self, cmd, args, $?)
   ensure
-    f.close if f
+    f.close if f and not f.closed?
     removed_ENV_variables.each do |key, value|
       ENV[key] = value
     end if removed_ENV_variables