fix error on multi resize/save calls

If we call resize + save multiple times without reopening the source
file resize will crash. The first save call clears the imagemagick
object and the second resize call triggers crash while calling
getImageWidth on imagemagick object.

Now the imagemagick object gets reinitialized after save.

This scenario can happen if you downscale multiple times with
Mage_Catalog_Helper_Image within templates like this:

	$helper = $this->helper('catalog/image');
	$helper->init($product, 'image', $file)
	echo (string) $helper->resize(200, 200);
	echo (string) $helper->resize(100, 100);