Fix UTF iterators end too early.

[Uhf7]

Fixes the bug in Utf16_Iter described here: #9599 (comment). I introduced this bug in 9599, it is a regression. Sorry for that. The problem is: If the last character of an UTF-16-coded file is greater then 0x7F, and hence needs more than one byte in UTF-8-encoding, only the 1st byte of the UTF-8 sequence arrives in the text buffer.

A similar bug does exist in Utf8_Iter, which is fixed too. This bug, at least, is no regression. It is also harder to reproduce. When writing an UTF-16-coded file, and

• the code point of the last character in the text buffer is above 0x0FFFF, which means, two 16-bit codes need to be written instead of one, and
• the position of the last character in the text buffer is 65536 (or any multiple of this),

then only the first 16-bit code is written to the file.

The 65536 comes from the size of the intermediate buffer which is used while conversion:

notepad-plus-plus/PowerEditor/src/Utf8_16.cpp

Line 344 in 6750be3

static const int bufSize = 64*1024;