fix: Limit memory allocation by MAX_CRYPTO_DATA_SIZE

[nickolay168]

This PR is intended to fix the issue, when shiny object is shown to the camera during the call. In this scenario camera creates big frames, for which toxcore tries to allocate memory and if the frame is big enough, it may fail, resulting in overflow error.

In this fix we limit the allocation size to MAX_CRYPTO_DATA_SIZE and, limit the frame size to UINT16_MAX (we use length_safe instead of length variable).

I have tested the fix on qTox client.

See issue

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This change is 

[Green-Sky]

@nickolay168 We appreciate your effort, but I found a better fix in #2768 . I hope we see more code changes from you :)

[nickolay168]

Sure! I will help with what I can.