Fix file descriptor leak in Formula#system

Library/Homebrew/formula.rb

@@ -591,7 +591,7 @@ class Formula
   rescue ErrorDuringExecution
     raise BuildError.new(self, cmd, args, $?)
   ensure
-    f.close if f and not f.closed?
+    [rd, f].each { |io| io.close if io and not io.closed? }
     ENV.update(removed_ENV_variables) if removed_ENV_variables
   end