media file: fix exception when getting frame count of corrupted gif.

Fix a `gifload: no frames in GIF` error from libvips when trying to read
the frame count for https://danbooru.donmai.us/media_assets/1141668.

app/logical/media_file/image.rb

@@ -40,7 +40,7 @@ class MediaFile::Image < MediaFile
   def frame_count
     case file_ext
     when :gif, :webp
-      image.get("n-pages") if image.get_fields.include?("n-pages")
+      n_pages
     when :png
       metadata.fetch("PNG:AnimationFrames", 1)
     when :avif
@@ -50,6 +50,13 @@ class MediaFile::Image < MediaFile
     end
   end
 
+  # @return [Integer, nil] The frame count for gif and webp images, or possibly nil if the file doesn't have a frame count or is corrupt.
+  def n_pages
+    image.get("n-pages")
+  rescue Vips::Error
+    nil
+  end
+
   def frame_rate
     return nil if !is_animated? || frame_count.nil? || duration.nil? || duration == 0
     frame_count / duration