Low coverage in CBMC proofs involving function pointers

Low coverage values are reported for some proofs that involve function pointers. CBMC considers values for them that are not reachable in practice (null coverage).

The proofs that present this behavior are:
* `aws_cryptosdk_enc_ctx_clean_up`
* `aws_cryptosdk_enc_ctx_clear`
* `aws_cryptosdk_serialize_frame`

Start by examining the CBMC proofs for aws_cryptosdk_enc_ctx_clean_up, aws_cryptosdk_enc_ctx_clear, and aws_cryptosdk_serialize_frame, then reproduce their reported coverage values. Determine how function-pointer values that are null or unreachable affect coverage, and consider the issue complete when these proofs report coverage that excludes those unreachable cases.

Written by the indexing model from the issue text.