Check that multibyte characters are valid using pm_strpbrk. We need
to add a couple of codepaths to ensure all encodings are covered.
Importantly this doesn't check regular expressions, because
apparently you're allowed to have invalid multibyte characters
inside regular expression comment groups/extended mode.
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* This is also faster than calling pm_encoding_utf_8_alpha_char/pm_encoding_utf_8_alnum_char
as those compute the character width and do extra checks.
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