check for a valid frame before using its shape

With the camera offline, no preview frame, and `camera-error.jpg` missing, `latest_frame` read `frame.shape` before its `frame is None` check, so it raised `AttributeError` and answered 500 with a traceback instead of the intended "Unable to get valid frame". The check now runs first.

frigate/api/media.py

@@ -258,15 +258,15 @@ async def latest_frame(
 
                 frame = request.app.camera_error_image
 
-        height = int(params.height or str(frame.shape[0]))
-        width = int(height * frame.shape[1] / frame.shape[0])
-
         if frame is None:
             return JSONResponse(
                 content={"success": False, "message": "Unable to get valid frame"},
                 status_code=500,
             )
 
+        height = int(params.height or str(frame.shape[0]))
+        width = int(height * frame.shape[1] / frame.shape[0])
+
         if height < 1 or width < 1:
             return JSONResponse(
                 content="Invalid height / width requested :: {} / {}".format(