Fix T90836: Strip snaps to empty space

This was caused by snap to hold offset feature, which calculates strip
content boundary, but it can be outside of strip boundary.

Clamp content start and end values so they are always inside of strip.

source/blender/editors/transform/transform_snap_sequencer.c

@@ -227,6 +227,10 @@ static void seq_snap_target_points_build(const TransInfo *t,
           content_end = seq->enddisp;
         }
       }
+
+      CLAMP(content_start, seq->startdisp, seq->enddisp);
+      CLAMP(content_end, seq->startdisp, seq->enddisp);
+
       snap_data->target_snap_points[i] = content_start;
       snap_data->target_snap_points[i + 1] = content_end;
       i += 2;