u8-istream: Fix handling of partial reads.

The u8-istream code did not retry upon a partial read, assuming that that
was the end of the file.  When the partial read was shorter than
ENCODING_GUESS_MIN, this could cause the encoding guesser, in turn, to
guess the wrong encoding (especially if the encoding was really UTF-16 and
the partial read was an odd number of bytes).

Reported at https://bugs.debian.org/760841.
Reported by Friedrich Beckmann and Steven Chamberlain.