Whenever the conv_c() function encounters an incomplete multibyte char,
it peeks ahead. It also sets p to peekbuf, to indicate it is still
processing the incomplete character.
However, on the next retry, it compares buf against peekbuf, which
always returns false, since both buf and peekbuf are local char arrays,
whose addresses are never the same.
Fix this by comparing against p instead, which was the intention. Also
turn peekbuf into an array of u_char, to prevent conversion warnings.
MFC after: 1 week
7818f8df2e