[intl] Fix empty-needle grapheme_strpos offsets - #263
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grapheme_strpos_utf16() returned raw UTF-16 code-unit positions for an empty needle instead of grapheme counts, so multi-code-unit graphemes made strpos()/strrpos() over-report the offset. Convert the boundary position like the non-empty search path; sibling audit found the ASCII fast paths and the strstr() UTF-16 position consumer are unaffected. Signed-off-by: Ilia Alshanetsky <ilia@ilia.ws>
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grapheme_strpos_utf16() handled an empty needle by returning the raw UTF-16 code-unit position of the search start (or the full UTF-16 haystack length for strrpos with a non-negative offset), while the non-empty path converts the boundary position back to a grapheme count, so for haystacks containing multi-code-unit graphemes like strpos() on a string with an emoji the functions reported UTF-16 offsets instead of grapheme offsets. The fix converts both empty-needle return values through grapheme_count_graphemes() exactly like the non-empty search path does; the ASCII fast paths are unaffected because byte and grapheme offsets coincide there, and grapheme_strstr() consumes the separate raw UTF-16 position which remains correct.