Uninitialized structs on the stack in AWS tasks

[gernst]

For example:

https://github.com/sosy-lab/sv-benchmarks/blob/b61ec96df19679bf5a65ec48ec1651ad2c8a0374/c/aws-c-common/aws_byte_buf_cat_harness.i#L9113

leading to undefined behavior in the test harness.