We must not free the read line, if the `READ_AHEAD` flag is set. This
also restores the expectations of SplFileObject_next_variation002.phpt.
Closes GH-7518.
As it is, `::seek(0)` sets the file pointer to the beginning of the
file, but `::seek($n)` where `$n > 0` sets the file pointer to the
beginning of the following line, having line `$n` already read into the
line buffer. This is pretty inconsistent; we fix it by always seeking
to the beginning of the line.
We also add a test case for the duplicate bug #46569.
Closes GH-6434.