common: Fix dead code in cockpit_json_walk_node()
common: Fix dead code in cockpit_json_walk_node()

Spotted by coverity:

    src/common/cockpitjson.c:1073: cond_notnull: Condition "copy", taking true branch. Now the value of "copy" is not "NULL".
    src/common/cockpitjson.c:1076: notnull: At condition "copy", the value of "copy" cannot be "NULL".
    src/common/cockpitjson.c:1076: dead_error_condition: The condition "copy" must be true.
    src/common/cockpitjson.c:1076: dead_error_line: Execution cannot reach the expression "json_object_ref(object)" inside this statement: "json_node_take_object(new_n...".

mvollmer/cockpit,martinpitt/cockpit,cockpit-project/cockpit,cockpit-project/cockpit,martinpitt/cockpit,garrett/cockpit,garrett/cockpit,cockpit-project/cockpit,mvollmer/cockpit,martinpitt/cockpit,mvollmer/cockpit,mvollmer/cockpit,garrett/cockpit,cockpit-project/cockpit,garrett/cockpit,cockpit-project/cockpit,martinpitt/cockpit,garrett/cockpit,martinpitt/cockpit,mvollmer/cockpit