fix potential null pointer dereference found by coverity (#106)
fix potential null pointer dereference found by coverity (#106)

CID 258040 (#9 of 9): Explicit null dereferenced (FORWARD_NULL)9. var_deref_model: Passing null pointer computed_args to ngx_http_echo_exec_echo_location, which dereferences it. [show details]
259            return ngx_http_echo_exec_echo_location(r, ctx, computed_args);
openresty/echo-nginx-module,openresty/echo-nginx-module,openresty/echo-nginx-module