Fix thinko when ending progress report for a backend

The logic ending progress reporting for a backend entry introduced by
b6fb6471 causes callers of pgstat_progress_end_command() to do some extra
work when track_activities is enabled as the process fields are reset in
the backend entry even if no command were started for reporting.

This resets the fields only if a command is registered for progress
reporting, and only if track_activities is enabled.

Author: Masahiho Sawada
Discussion: https://postgr.es/m/CAD21AoCry_vJ0E-m5oxJXGL3pnos-xYGCzF95rK5Bbi3Uf-rpA@mail.gmail.com
Backpatch-through: 9.6

src/backend/postmaster/pgstat.c

@@ -3272,10 +3272,10 @@ pgstat_progress_end_command(void)
 {
 	volatile PgBackendStatus *beentry = MyBEEntry;
 
-	if (!beentry)
+	if (!beentry || !pgstat_track_activities)
 		return;
-	if (!pgstat_track_activities
-		&& beentry->st_progress_command == PROGRESS_COMMAND_INVALID)
+
+	if (beentry->st_progress_command == PROGRESS_COMMAND_INVALID)
 		return;
 
 	PGSTAT_BEGIN_WRITE_ACTIVITY(beentry);