[RLlib] Fix bug in MultiAgentEnv when restarting a sub-env (index error).

[sven1977]

Fix bug in MultiAgentEnv when restarting a sub-env (index error).

Why are these changes needed?

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    method in Tune, I've added it in doc/source/tune/api/ under the
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• Testing Strategy
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  • This PR is not tested :(

[simonsays1980]

LGTM :)