getprogname on posix: fix locking (mutex was not unlocked in some cases)

src/platforms/posix/px4_layer/px4_posix_tasks.cpp

@@ -391,16 +391,19 @@ unsigned long px4_getpid()
 const char *getprogname()
 {
 	pthread_t pid = pthread_self();
+	const char *prog_name = "UnknownApp";
+
+	pthread_mutex_lock(&task_mutex);
 
 	for (int i = 0; i < PX4_MAX_TASKS; i++) {
 		if (taskmap[i].isused && taskmap[i].pid == pid) {
-			pthread_mutex_lock(&task_mutex);
-			return taskmap[i].name.c_str();
-			pthread_mutex_unlock(&task_mutex);
+			prog_name = taskmap[i].name.c_str();
 		}
 	}
 
-	return "Unknown App";
+	pthread_mutex_unlock(&task_mutex);
+
+	return prog_name;
 }
 
 int px4_prctl(int option, const char *arg2, unsigned pid)