improve simple palette detection for PNG

- reverts change #1239 which could remove transparency from valid images
  (see test_save_p_transparent_black test case)
- improves simple palette detection to handle images from #1238

PIL/PngImagePlugin.py

@@ -74,8 +74,7 @@ _MODES = {
 }
 
 
-_simple_palette = re.compile(b'^\xff+\x00\xff*$')
-_null_palette = re.compile(b'^\x00*$')
+_simple_palette = re.compile(b'^\xff*\x00\xff*$')
 
 # Maximum decompressed size for a iTXt or zTXt chunk.
 # Eliminates decompression bombs where compressed chunks can expand 1000x
@@ -359,12 +358,14 @@ class PngStream(ChunkStream):
         s = ImageFile._safe_read(self.fp, length)
         if self.im_mode == "P":
             if _simple_palette.match(s):
+                # tRNS contains only one full-transparent entry,
+                # other entries are full opaque
                 i = s.find(b"\0")
                 if i >= 0:
                     self.im_info["transparency"] = i
-            elif _null_palette.match(s):
-                self.im_info["transparency"] = 0
             else:
+                # otherwise, we have a byte string with one alpha value
+                # for each palette entry
                 self.im_info["transparency"] = s
         elif self.im_mode == "L":
             self.im_info["transparency"] = i16(s)