properly check if pdf and svg modules are installed

lib/private/preview/office.php

@@ -6,7 +6,7 @@
  * See the COPYING-README file.
  */
 //both, libreoffice backend and php fallback, need imagick
-if (extension_loaded('imagick')) {
+if (extension_loaded('imagick') && count(\Imagick::queryFormats("PDF")) === 1) {
 	$isShellExecEnabled = \OC_Helper::is_function_enabled('shell_exec');
 
 	// LibreOffice preview is currently not supported on Windows

lib/private/preview/pdf.php

@@ -7,7 +7,7 @@
  */
 namespace OC\Preview;
 
-if (extension_loaded('imagick')) {
+if (extension_loaded('imagick') && count(\Imagick::queryFormats("PDF")) === 1) {
 
 	class PDF extends Provider {
 

lib/private/preview/svg.php

@@ -7,7 +7,7 @@
  */
 namespace OC\Preview;
 
-if (extension_loaded('imagick')) {
+if (extension_loaded('imagick') && count(\Imagick::queryFormats("SVG")) === 1) {
 
 	class SVG extends Provider {
 

lib/private/preview/unknown.php

@@ -22,7 +22,7 @@ class Unknown extends Provider {
 
 		$svgPath = substr_replace($path, 'svg', -3);
 
-		if (extension_loaded('imagick') && file_exists($svgPath)) {
+		if (extension_loaded('imagick') && file_exists($svgPath) && count(\Imagick::queryFormats("SVG")) === 1) {
 
 			// http://www.php.net/manual/de/imagick.setresolution.php#85284
 			$svg = new \Imagick();