decoder/h264: consolidate code modifying picture to one place

src/decoder/stateless/h264.rs

@@ -1157,13 +1157,11 @@ where
         let max_frame_num = sps.max_frame_num() as i32;
 
         let mut pic = PictureData::new_from_slice(slice, &sps, timestamp);
-
+        self.codec.compute_pic_order_count(&mut pic, &sps)?;
         if let Some(first_field) = first_field {
             pic.set_first_field_to(first_field);
         }
 
-        self.codec.compute_pic_order_count(&mut pic, &sps)?;
-
         if matches!(pic.is_idr, IsIdr::Yes { .. }) {
             // C.4.5.3 "Bumping process"
             // The bumping process is invoked in the following cases: