Fix #80933: SplFileObject::DROP_NEW_LINE is broken for NUL and CR

`buf` may contain NUL bytes, so we must not use `strcspn()` but rather
a binary safe variant. However, we also must not

Fix #80933: SplFileObject::DROP_NEW_LINE is broken for NUL and CR

`buf` may contain NUL bytes, so we must not use `strcspn()` but rather
a binary safe variant. However, we also must not detect a stray CR as
line ending, and since we only need to check line endings at the end
of the buffer, we can nicely optimize.

Co-authored-by: Nikita Popov <nikita.ppv@gmail.com>

Closes GH-6836.

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