Check that dynamic array length does not exceed overall message length

Rammbock blindly believes the length of a dynamic array. It does not even check if it fits to overall message length. This lead into a hard-to debug problem where the memory of Python virtual machine was exhausted when the production code left the dynamic message length field uninitialized.

There should be at least a check for dynamic array size being smaller than overall message length. Probably also a global limit for message length which one could optionally set when importing Rammbock.

Start by tracing the dynamic-array length handling and how the overall message length is used before allocation or parsing. Confirm how an uninitialized length can exhaust memory, then define done as rejecting lengths beyond the message size; the optional global message-length limit remains an unresolved design question.

Written by the indexing model from the issue text.