[3.12] gh-95782: Fix io.BufferedReader.tell() etc. being able to return offsets < 0 (GH-99709) (GH-115599)

lseek() always returns 0 for character pseudo-devices like
`/dev/urandom` (for other non-regular files, e.g. `/dev/stdin`, it
always returns -1, to which CPython reacts by raising appropriate
exceptions). They are thus technically seekable despite not having seek
semantics.

When calling read() on e.g. an instance of `io.BufferedReader` that
wraps such a file, `BufferedReader` reads ahead, filling its buffer,
creating a discrepancy between the number of bytes read and the internal
`tell()` always returning 0, which previously resulted in e.g.
`BufferedReader.tell()` or `BufferedReader.seek()` being able to return
positions < 0 even though these are supposed to be always >= 0.

Invariably keep the return value non-negative by returning
max(former_return_value, 0) instead, and add some corresponding tests.
(cherry picked from commit 26800cf25a)

Co-authored-by: 6t8k <58048945+6t8k@users.noreply.github.com>

Lib/_pyio.py

@@ -1209,7 +1209,8 @@ class BufferedReader(_BufferedIOMixin):
         return written
 
     def tell(self):
-        return _BufferedIOMixin.tell(self) - len(self._read_buf) + self._read_pos
+        # GH-95782: Keep return value non-negative
+        return max(_BufferedIOMixin.tell(self) - len(self._read_buf) + self._read_pos, 0)
 
     def seek(self, pos, whence=0):
         if whence not in valid_seek_flags: