Update secure_img_handler.php

1 - Prevent secure_img_handler.php from sending different Content-Type Header to the image when including other php file (example e107_config.php);
in some case not clearing the buffer cause the secure image to not reander properly since we send two different content type header.

2 - Also added imagedestroy() to free additional memory.

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NOTE: found this bug on Apache server with PHP Version 5.2.17