batch: Fix crash caused by calling post_func on unused batch entries

A null pointer dereference could happen when running with more threads
than there were rows in the image, leading to denial of service (crash).
Reported by @Han0nly via huntr.dev.

chafa/internal/chafa-batch.c

@@ -82,7 +82,12 @@ chafa_process_batches (gpointer ctx, GFunc batch_func, GFunc post_func, gint n_r
         }
 
         if (row_ofs [0] >= row_ofs [1])
+        {
+            /* Save the number of batches actually produced to use in
+             * post_func loop later. */
+            n_batches = i;
             break;
+        }
 
         batch = &batches [i++];
         batch->first_row = row_ofs [0];