[HarfBuzz] Infinite loop during decomposition when icu_unicode_funcs is used

[Kenichi Ishibashi]

Hi,

hb_icu_unicode_decompose() uses ICU's u_strlen() to get the number of
Unicode codepoints in normalized buffer. However, it seems that it returns
the number of UChars in the buffer. UChar is equivalent to uint16_t. This
means that we can't get right number of codepoints when the buffer contains
surrogate pairs. This eventually causes infinite loop during decomposition.
For example, if the function is called like:

  hb_codepoint_t a, b;
  hb_icu_unicode_decompose(0/*unused*/, 0x1f1ef /* REGIONAL INDICATOR SYMBOL
LETTER J */, &a, &b, 0/*unused*/);

then, it returns TRUE with *a == 0x1f1ef. This leads infinite loop in
decompose(). Attached patch would fix the problem.

Thanks,
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