Expose invalid/corrupt image count rule in the ImageInfo Plugin

[bitsikas]

We could add an eval rule for invalid/corrupt images.

We already store the total count and the per image subtype count in the local variables of the plugin (set_local/get_local) so what's needed is the actual eval rule that checks them

as parameters it would need the type of image or "all" and the min/max count we want to check for

example rules

body     MIN_THREE_CORRUPTED                     eval:image_invalid_count('all','3')
body     BETWEEN_3_AND_5_CORRUPTED_GIFS          eval:image_invalid_count('gif','3','5')