Silence compiler warning

My original coding was questionable anyway.

Reported-by: Sergei Kornilov
Discussion: https://postgr.es/m/9645101543575886@myt6-27270b78ac4f.qloud-c.yandex.net

src/backend/tcop/postgres.c

@@ -2244,12 +2244,13 @@ check_log_duration(char *msec_str, bool was_logged)
 		/*
 		 * Do not log if log_statement_sample_rate = 0. Log a sample if
 		 * log_statement_sample_rate <= 1 and avoid unecessary random() call
-		 * if log_statement_sample_rate = 1.
+		 * if log_statement_sample_rate = 1.  But don't compute any of this
+		 * unless needed.
 		 */
-		if (exceeded)
-			in_sample = log_statement_sample_rate != 0 &&
-				(log_statement_sample_rate == 1 ||
-				 random() <= log_statement_sample_rate * MAX_RANDOM_VALUE);
+		in_sample = exceeded &&
+			log_statement_sample_rate != 0 &&
+			(log_statement_sample_rate == 1 ||
+			 random() <= log_statement_sample_rate * MAX_RANDOM_VALUE);
 
 		if ((exceeded && in_sample) || log_duration)
 		{