Initialise __frame = 0 in open, and test tell

Tests/test_file_png.py

@@ -629,6 +629,10 @@ class TestFilePng:
         with Image.open(test_file) as reloaded:
             assert reloaded.info["exif"] == b"Exif\x00\x00exifstring"
 
+    def test_tell(self, tmp_path):
+        with Image.open(TEST_PNG_FILE) as im:
+            assert im.tell() == 0
+
     def test_seek(self, tmp_path):
         with Image.open(TEST_PNG_FILE) as im:
             im.seek(0)

src/PIL/PngImagePlugin.py

@@ -630,13 +630,13 @@ class PngImageFile(ImageFile.ImageFile):
 
     format = "PNG"
     format_description = "Portable network graphics"
-    __frame = 0
 
     def _open(self):
 
         if self.fp.read(8) != _MAGIC:
             raise SyntaxError("not a PNG file")
         self.__fp = self.fp
+        self.__frame = 0
 
         #
         # Parse headers up to the first IDAT or fDAT chunk