Fix threading issue that at best will leak memory

The problem is the checking in policy_cache_set, there is a race
condition between the null check and obtaining the lock.  The fix is in
policy_cache_new to detect if the creation has happened already.

Reviewed-by: Richard Levitte <levitte@openssl.org>

crypto/x509v3/pcy_cache.c

@@ -78,6 +78,9 @@ static int policy_cache_new(X509 *x)
     CERTIFICATEPOLICIES *ext_cpols = NULL;
     POLICY_MAPPINGS *ext_pmaps = NULL;
     int i;
+
+    if (x->policy_cache != NULL)
+        return 1;
     cache = OPENSSL_malloc(sizeof(*cache));
     if (cache == NULL)
         return 0;