The ornamental design for a ventilator, as shown and described.
1. Ventilator
1.1 : Front
1.2 : Back
1.3 : Left
1.4 : Right
1.5 : Top
1.6 : Bottom
1.7 : Perspective
1.1 is a front elevation view of a ventilator showing our new design; 1.2 is a rear elevation view thereof; 1.3 is a left side elevation view thereof; 1.4 is a right side elevation view thereof; 1.5 is a top plan view thereof; 1.6 is a bottom plan view thereof; and 1.7 is a perspective view thereof; the present product is used for exchange between indoor and outdoor air and air purification.
The broken lines in Figures 1.1-1.7 depict portions of the boiling pot that form no part of the claimed design.