Remove panic() call from the unexpected relinquish execution

As this library is designed to be integrated into other software, we should avoid using panic(). Oterwise it may have really bad outcomes.

For example, in GitLab Runner this library will be used to handle autoscaling of jobs execution. If at any moment inst.acquired would go bellow 0 - even if such case is nearly impossible to happen - a panic() call would cause GitLab Runner process failure and termination. This means that connection to all executed jobs would be lost and they would be left in undefined state.

Unexpected or invalid situations should be either logged and handled gracefuly internally in the library or should be returned as an error to the caller.