Update alg_props allocation to use ensure_... function

[adpaco-aws]

alg_props structures are commonly initialized as

struct aws_cryptosdk_alg_properties alg_props;

ensure_alg_properties_attempt_allocation(&alg_props);

but we have updated ensure_alg_properties_attemp_allocation in #638 . Thus we should change the previous code to

struct aws_cryptosdk_alg_properties alg_props = ensure_alg_properties_attempt_allocation(MAX_STRING_LEN);

in all proofs using alg_props structures.