libpq: Fix PQtrace() format for non-printable characters
PQtrace() was generating its output for non-printable characters without
casting the characters printed with unsigned char, leading to some extra
"\xffffff" generated in the output due to the fact that char may be
signed.
Oversights introduced by commit
198b3716dba6, so backpatch down to v14.
Author: Ran Benita <ran@unusedvar.com>
Discussion: https://postgr.es/m/
a3383211-4539-459b-9d51-
95c736ef08e0@app.fastmail.com
Backpatch-through: 14