Use an extra assert! to avoid bounds checks.

This change has been suggested by @okaneco in image-rs#414 (comment) - thanks!

Co-authored-by: Collyn O'Kane <[email protected]>

src/filter.rs

@@ -119,6 +119,7 @@ mod simd {
             prev_row = &prev_row[3..];
             curr_row = &mut curr_row[3..];
         }
+        assert!(prev_row.len() >= 3 && curr_row.len() >= 3);
         // Can't use `u8x4::from_slice` for the last `[u8;3]`.
         let b = load3(prev_row);
         let mut x = load3(curr_row);