When loading a file via `FileManager::loadFileData`, a fixed-length buffer
is filled via `fread`. Then, in some cases, a conversion is done with the help
of `Utf8_16_Read`. However, the method `Utf8_16_Read::convert` performs a call
to `strlen` on this buffer. This is obviously wrong: `\0` char should be
accepted (even if a bit strange) and the buffer is not zero-terminated.
The changes merely consist in adding an additional parameter `length` to
not have to guess the size of the buffer.
