Reduce number of bytes read for PCX header (#9004)

Author: hugovk

src/PIL/PcxImagePlugin.py

@@ -54,7 +54,7 @@ def _open(self) -> None:
         # header
         assert self.fp is not None
 
-        s = self.fp.read(128)
+        s = self.fp.read(68)
         if not _accept(s):
             msg = "not a PCX file"
             raise SyntaxError(msg)
@@ -66,6 +66,8 @@ def _open(self) -> None:
             raise SyntaxError(msg)
         logger.debug("BBox: %s %s %s %s", *bbox)
 
+        offset = self.fp.tell() + 60
+
         # format
         version = s[1]
         bits = s[3]
@@ -102,7 +104,6 @@ def _open(self) -> None:
                         break
                 if mode == "P":
                     self.palette = ImagePalette.raw("RGB", s[1:])
-            self.fp.seek(128)
 
         elif version == 5 and bits == 8 and planes == 3:
             mode = "RGB"
@@ -128,9 +129,7 @@ def _open(self) -> None:
         bbox = (0, 0) + self.size
         logger.debug("size: %sx%s", *self.size)
 
-        self.tile = [
-            ImageFile._Tile("pcx", bbox, self.fp.tell(), (rawmode, planes * stride))
-        ]
+        self.tile = [ImageFile._Tile("pcx", bbox, offset, (rawmode, planes * stride))]
 
 
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