Update aws_byte_buf_is_bounded to include a null check

This would avoid having the following code in proofs:
```
__CPROVER_assume(buf1 != NULL);
__CPROVER_assume(aws_byte_buf_is_bounded(buf1, MAX_BUFFER_SIZE));
ensure_byte_buf_has_allocated_buffer_member(buf1);
__CPROVER_assume(aws_byte_buf_is_valid(buf1));
```
by removing the first line (which should be done in `aws_byte_buf_is_bounded` in any case)

Locate aws_byte_buf_is_bounded and the proof snippet in the issue, then inspect the function's current precondition handling and its callers. Done means the function accounts for a null buffer so the explicit __CPROVER_assume(buf1 != NULL) is no longer needed, with existing tests or proofs still passing.

Written by the indexing model from the issue text.