backdrop: avoid a (benign) zero-sized read

Found with MEM_DEBUG added in later change.

Signed-off-by: Elias Naur <mail@eliasnaur.com>

piet-gpu/shader/backdrop.comp

@@ -85,10 +85,11 @@ void main() {
                 el_ix = probe;
             }
         }
-        uint seq_ix = row - (el_ix > 0 ? sh_row_count[el_ix - 1] : 0);
         uint width = sh_row_width[el_ix];
+        if (width > 0) {
             // Process one row sequentially
             // Read backdrop value per tile and prefix sum it
+            uint seq_ix = row - (el_ix > 0 ? sh_row_count[el_ix - 1] : 0);
             uint tile_el_ix = sh_row_base[el_ix] + seq_ix * 2 * width;
             uint sum = memory[tile_el_ix];
             for (uint x = 1; x < width; x++) {
@@ -97,4 +98,5 @@ void main() {
                 memory[tile_el_ix] = sum;
             }
         }
+    }
 }