Fix resource leaks identified by cppcheck

* Where possible rearrange error checks to avoid branching
* Otherwise add missing `fclose`, `close`, `px4_close` calls before return

src/systemcmds/tests/test_ppm_loopback.c

@@ -79,6 +79,8 @@ int test_ppm_loopback(int argc, char *argv[])
 
 	if (result != OK) {
 		warnx("PWM_SERVO_GET_COUNT");
+
+		(void)close(servo_fd);
 		return ERROR;
 	}