Fix #81693: mb_check_encoding(7bit) segfaults

`php_mb_check_encoding()` now uses conversion to `mbfl_encoding_wchar`.
Since `mbfl_encoding_7bit` has no `input_filter`, no filter can be

Fix #81693: mb_check_encoding(7bit) segfaults

`php_mb_check_encoding()` now uses conversion to `mbfl_encoding_wchar`.
Since `mbfl_encoding_7bit` has no `input_filter`, no filter can be
found. Since we don't actually need to convert to wchar, we encode to
8bit.

Closes GH-7712.

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