progress: call trace2_region_leave() only after calling _enter()

A user of progress API calls start_progress() conditionally and
depends on the display_progress() and stop_progress() functions to
become no-op when start_progress() hasn't been called.

As we added a call to trace2_region_enter() to start_progress(), the
calls to other trace2 API calls from the progress API functions must
make sure that these trace2 calls are skipped when start_progress()
hasn't been called on the progress struct.  Specifically, do not
call trace2_region_leave() from stop_progress() when we haven't
called start_progress(), which would have called the matching
trace2_region_enter().

Signed-off-by: Derrick Stolee <dstolee@microsoft.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

progress.c

@@ -329,13 +329,9 @@ void stop_progress(struct progress **p_progress)
 			trace2_data_intmax("progress", the_repository,
 					   "total_bytes",
 					   (*p_progress)->throughput->curr_total);
-	}
 
-	trace2_region_leave("progress",
-			    p_progress && *p_progress
-				? (*p_progress)->title
-				: NULL,
-			    the_repository);
+		trace2_region_leave("progress", (*p_progress)->title, the_repository);
+	}
 
 	stop_progress_msg(p_progress, _("done"));
 }