Initialise __logical_frame = 0 so tell() == 0 when unpickled

Co-Authored-By: Andrew Murray <3112309+radarhere@users.noreply.github.com>

Author: hugovk

src/PIL/WebPImagePlugin.py

@@ -39,7 +39,7 @@ class WebPImageFile(ImageFile.ImageFile):
     format = "WEBP"
     format_description = "WebP image"
     __loaded = -1
-    __logical_frame = -1
+    __logical_frame = 0
 
     def _open(self):
         if not _webp.HAVE_WEBPANIM: