This enables it to handle data that isn't presented as a
NUL-terminated string.
In particular, the NUL byte can appear _within_ the string and be
correctly translated to the NUL wide character. So I've been able to
remove the awkwardness in the test rig of having to include the
terminating NUL in every test to ensure NUL has been tested, and
instead, insert a single explicit test for it.
Similarly to the previous commit, the simplification at the (one) call
site gives me a strong feeling of 'this is what the API should have
been all along'!
The test in question was supposed to contain the spurious UTF-8
encoding that 0xD800 would have if it were not a surrogate. But the
final continuation character 0x80 was instead 0x00.
The test passed anyway, because ED A0 was regarded as a truncated
sequence, instead of ED A0 80 being regarded as an illegal encoding of
a surrogate, and both return the same output!
