ggml rpc : Add missing check for rpc buffer type

[struct]

This adds a missing check for ggml_backend_buffer_is_rpc in serialize_tensor. When using the rpc server, the backend might pass a buffer to serialize_tensor via ggml_backend_rpc_buffer_type_get_alloc_size which incorrectly assumes it is an rpc buffer and casts a pointer (ctx) of type ggml_backend_rpc_buffer_context to it. In practice I think this just results in a nullptr crash.

test-backend-ops completes successfully