regcomp utf8 len cache panic

regcomp utf8 len cache panic

Compiling this regex:

    /\x{100}[xy]\x{100}{2}/

caused this:

    panic: sv_len_utf8 cache 1 real 2

This was due to the code in S_study_chunk() mixing up char and byte
lengths when updating the utf8 length cache on a utf8 string that
had been extended by repeatedly duplicating the last n chars.

(The second test is for an issue introduced during an initial attempt to
fix this).