utf8.c: Add utf8_to_uvchr_buf() and utf8_to_uvuni_buf()

utf8.c: Add utf8_to_uvchr_buf() and utf8_to_uvuni_buf()

The existing functions (utf8_to_uvchr and utf8_to_uvuni) have a
deficiency in that they could read beyond the end of the input string if
given malformed input.  This commit creates two new functions which
behave as the old ones did, but have an extra parameter each, which
gives the upper limit to the string, so no read beyond it is done.