utf8.c: Slight code simplification

utf8.c: Slight code simplification

The changed code computes the code point to output in a warning.
However, the function it calls will output essentially the same warning
if called with the proper flag.  So, just do that.  The slight change in
the output is that the calling function adds the operation name, like
"in print" to the end of the message, so this changes causes a better
message to be output.  I did not change the handling of surrogates here,
because the warning messages are too different.