Here are two new patch. Checking for characters out-of-range moved,
making the code simpler. Theoretically it is a bit slow down decoding of
short UCS1 strings (up to 1 and 3 chars on 32- and 64-bit), but
practically there is no difference. The second patch is different from
the first patch that masks are not calculated and specified explicitly.
I am not sure that it improves readability. The commiter has the choice.