Make crypto_free_pk_env tolerate NULL arg in 0.2.1. Error-proofing against bug 988 backport

src/common/crypto.c

@@ -321,7 +321,8 @@ crypto_new_pk_env(void)
 void
 crypto_free_pk_env(crypto_pk_env_t *env)
 {
-  tor_assert(env);
+  if (!env)
+    return;
 
   if (--env->refs > 0)
     return;