Check if a preview provider is available before using it

Else if a preview provider is registerd but not available (for example
missing support in some external lib). It will do 💥. This way the
providers can at least do the sanity checks required.

Signed-off-by: Roeland Jago Douma <roeland@famdouma.nl>

lib/private/Preview/Generator.php

@@ -184,6 +184,10 @@ private function getMaxPreview(ISimpleFolder $previewFolder, File $file, $mimeTy
 					continue;
 				}
 
+				if (!$provider->isAvailable($file)) {
+					continue;
+				}
+
 				$maxWidth = (int)$this->config->getSystemValue('preview_max_x', 4096);
 				$maxHeight = (int)$this->config->getSystemValue('preview_max_y', 4096);