Fix SSL_get0_chain_certs.

SSL_get0_chain_certs calls a ctrl function with
SSL_CTRL_GET_CHAIN_CERTS. The switch failed to set a positive return
value and so the call always appeared to fail.

Change-Id: If40ca7840197a9748fd69b761fd905f44bb79835
Reviewed-on: https://boringssl-review.googlesource.com/4521
Reviewed-by: Adam Langley <agl@google.com>