**What**
* Updated `copy_buffer_to_iovec_app` so that it copies as much of the buffer into the iovec as specified
* Throw invalid value when allocating an iovec of size 0
**Why**
* A bug found from TCP client example which allocates 1024 for the iovec size (where the buf size is also 1024) but received bytes is passed in as the `buf_size` argument to `copy_buffer_to_iovec_app`. This would return early after hitting this check `buf + data->buf_len > buf_begin + buf_size`. However, if the amount to copy is less than the iovec size, we should copy that much of the buf size. Eg TCP client sample receives 27(?) bytes at a time, and this copies 27 bytes into the iovec of size 1024
* The TCP client example attempts to recv bytes of size 0, this attempts to wasm malloc size 0, which outputs a warning. We should early return if recv bytes of size 0
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