Denial of service by double-checked locking in openssl-src

If an X.509 certificate contains a malformed policy constraint and policy processing is enabled, then a write lock will be taken twice recursively. On some operating systems (most widely: Windows) this results in a denial of service when the affected process hangs. Policy processing being enabled on a publicly facing server is not considered to be a common setup. Policy processing is enabled by passing the -policy' argument to the command line utilities or by calling either X509_VERIFY_PARAM_add0_policy()' or `X509_VERIFY_PARAM_set1_policies()' functions.