[Bug][server-api] copy the node ,the preTasks is error

[wangxj3]

When we copy a node form process definition,the value preTasks is the source node .when we link the noe previous node ,this value would't change .when start a process instance from fail node ,this error may cause that some copy node which agter the failed node will not execute.

[ruanwenjun]

Yeah, there is a problem, and I will try to fix.