Small patch to getCoverImageUrl to handle missing imagick library

I needed to deploy this Plugin on a fairly outdated server, which did not support the imagick extension. Since the current version does *not* check if imagick is actually installed, it would simply crash.

Here’s a quick fix to first check for the presence of imagick before trying to generate the png image:

function getCoverImageUrl($doc) {
		   $filename = FILE_CACHE_DIR . $doc->id . ".png";
		   if (file_exists($filename)) {
		      return FILE_CACHE_URL . $doc->id . ".png";
		   }
		   $filenamepdf = FILE_CACHE_DIR . $doc->id . ".pdf";
		   if (!file_exists($filenamepdf)) {
		      return null;
		   }
		   // PATCH TO HANDLE NON-INSTALL IMAGIC EXTENSION
		   if (!extension_loaded('imagick')) {
        			return null;
		   }
		   // PATCH END

		   // generate png ...
		   $im = new imagick($filenamepdf."[0]");
                   $im->resampleImage (10, 10, imagick::FILTER_UNDEFINED,1);
                   $im->setCompressionQuality(80);
                   $im->setImageFormat('png');
                   $im->writeImage($filename);
                   $im->clear();
                   $im->destroy();
		   return FILE_CACHE_URL . $doc->id . ".png";
		}

Maybe this is useful enough to include it in the next update?

Cheers,

-m

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