ggml : fix ggml_cast (ggml/973)

ggml/src/ggml.c

@@ -5818,6 +5818,7 @@ struct ggml_tensor * ggml_cast(
 
     result->op     = GGML_OP_CPY;
     result->src[0] = a;
+    result->src[1] = result;
 
     return result;
 }