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 198b3716db, 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

src/interfaces/libpq/fe-trace.c

@@ -112,7 +112,7 @@ pqTraceOutputByte1(FILE *pfdebug, const char *data, int *cursor)
 	 * that completes ErrorResponse and NoticeResponse messages.
 	 */
 	if (!isprint((unsigned char) *v))
-		fprintf(pfdebug, " \\x%02x", *v);
+		fprintf(pfdebug, " \\x%02x", (unsigned char) *v);
 	else
 		fprintf(pfdebug, " %c", *v);
 	*cursor += 1;
@@ -200,7 +200,7 @@ pqTraceOutputNchar(FILE *pfdebug, int len, const char *data, int *cursor)
 		else
 		{
 			fwrite(v + next, 1, i - next, pfdebug);
-			fprintf(pfdebug, "\\x%02x", v[i]);
+			fprintf(pfdebug, "\\x%02x", (unsigned char) v[i]);
 			next = i + 1;
 		}
 	}