[C][Client] Fix coredump when releasing the memory of an incompleted model

[ityuhui]

Sometimes, the model is not created completely due to some error (e.g. JSON string of a data field of the model cannot be parsed). Only a part of model is created.

At this time, we need check whether the pointer pointing to the data field is valid to avoid core-dump before releasing its memory.

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@wing328 @zhemant @michelealbano

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[wing328]

CircleCI failure not related to this change.

[ityuhui]

Thank you @agilob @wing328