Fix out of bounds offset handling with empty needle

For strrpos with positive out of bounds offsets was not detected.

Normalize substr() behavior

Make the behavior of substr(), mb_substr(), iconv_substr() and
grapheme_substr() consistent when it comes to the handling of
out of bounds offsets. substr

Normalize substr() behavior

Make the behavior of substr(), mb_substr(), iconv_substr() and
grapheme_substr() consistent when it comes to the handling of
out of bounds offsets. substr() will now always clamp out of
bounds offsets to the string boundary. Cases that previously
returned false will now return an empty string. This means that
substr() itself *always* returns a string now (like mb_substr()
already did before.)

Closes GH-6182.

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Handle out-of-bounds offset consistently in grapheme_* API

Make sure we throw the same kind of error regardless of whether
the offset is out-of-bounds in the fast path or in the slow

Handle out-of-bounds offset consistently in grapheme_* API

Make sure we throw the same kind of error regardless of whether
the offset is out-of-bounds in the fast path or in the slow
path.

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Fix grapheme out of bounds check

An offset equal to the string length is not out of bounds.