Fix md_decode_utf16le_before__() return value

[MakotoE]

I believe md_decode_utf16le_before__() returns the wrong value. The function is supposed to return the character before the offset. We can compare with the utf8 version md_decode_utf8_before__() which returns CH(off-1).

Closes https://github.com/mity/md4c/issues/275

[s77rt]

Test case that this PR fixes: **text**abc Expected results: text to be in bold