Fix memleak in s2n_x509_validator_validate_cert_chain (#733)
Fix memleak in s2n_x509_validator_validate_cert_chain (#733)

Previously was using X509_STORE_CTX_cleanup, which cleans up structure,
but doesn't free memory for it.
wcs1only/s2n,colmmacc/s2n,wcs1only/s2n,gibson-compsci/s2n,raycoll/s2n,PKRoma/s2n,wcs1only/s2n,bpdavidson/s2n,PKRoma/s2n,bpdavidson/s2n,raycoll/s2n,alexeblee/s2n,wcs1only/s2n,wcs1only/s2n,colmmacc/s2n,colmmacc/s2n,awslabs/s2n,PKRoma/s2n,awslabs/s2n,raycoll/s2n,bpdavidson/s2n,wcs1only/s2n,PKRoma/s2n,gibson-compsci/s2n,awslabs/s2n,gibson-compsci/s2n,awslabs/s2n,raycoll/s2n,colmmacc/s2n,colmmacc/s2n,alexeblee/s2n,gibson-compsci/s2n,PKRoma/s2n,wcs1only/s2n,alexeblee/s2n,raycoll/s2n,colmmacc/s2n,PKRoma/s2n,gibson-compsci/s2n,alexeblee/s2n,raycoll/s2n,wcs1only/s2n,PKRoma/s2n,alexeblee/s2n,awslabs/s2n,PKRoma/s2n,bpdavidson/s2n,bpdavidson/s2n,awslabs/s2n,gibson-compsci/s2n,alexeblee/s2n,bpdavidson/s2n