fix pnnx crash on unsupported expression

tools/pnnx/src/pass_level2/torch_mean.cpp

@@ -40,4 +40,26 @@ pnnx.Output             output      1 0 out
 
 REGISTER_GLOBAL_PNNX_GRAPH_REWRITER_PASS(torch_mean, 20)
 
+class torch_mean_1 : public GraphRewriterPass
+{
+public:
+    const char* match_pattern_graph() const
+    {
+        return R"PNNXIR(7767517
+4 3
+pnnx.Input              input_0     0 1 input
+prim::Constant          op_0        0 1 dtype value=*
+aten::mean              op_1        2 1 input dtype out
+pnnx.Output             output      1 0 out
+)PNNXIR";
+    }
+
+    const char* type_str() const
+    {
+        return "torch.mean";
+    }
+};
+
+REGISTER_GLOBAL_PNNX_GRAPH_REWRITER_PASS(torch_mean_1, 20)
+
 } // namespace pnnx

tools/pnnx/src/pass_level2/torch_sum.cpp

@@ -40,4 +40,26 @@ pnnx.Output             output      1 0 out
 
 REGISTER_GLOBAL_PNNX_GRAPH_REWRITER_PASS(torch_sum, 20)
 
+class torch_sum_1 : public GraphRewriterPass
+{
+public:
+    const char* match_pattern_graph() const
+    {
+        return R"PNNXIR(7767517
+4 3
+pnnx.Input              input_0     0 1 input
+prim::Constant          op_0        0 1 dtype value=*
+aten::sum               op_1        2 1 input dtype out
+pnnx.Output             output      1 0 out
+)PNNXIR";
+    }
+
+    const char* type_str() const
+    {
+        return "torch.sum";
+    }
+};
+
+REGISTER_GLOBAL_PNNX_GRAPH_REWRITER_PASS(torch_sum_1, 20)
+
 } // namespace pnnx

tools/pnnx/src/pass_ncnn/expand_expression.cpp

@@ -265,6 +265,13 @@ void expand_expression(Graph& graph)
             Operand* old_output_operand = op->outputs[0];
             Operand* new_output_operand = graph.get_operand(op->name + "_" + outname);
 
+            if (!new_output_operand)
+            {
+                // not supported expr
+                nonsupported_expr_ops.insert(op);
+                break;
+            }
+
             for (auto r : op->inputs)
             {
                 r->remove_consumer(op);