SSL: fixed possible configuration overwrite loading "engine:" keys.

When loading certificate keys via ENGINE_load_private_key() in runtime,
it was possible to overwrite configuration on ENGINE_by_id() failure.
OpenSSL documention doesn't describe errors in details, the only reason
I found in the comment to example is when the engine is not available.

src/event/ngx_event_openssl.c

@@ -764,13 +764,13 @@ ngx_ssl_load_certificate_key(ngx_pool_t *pool, char **err,
 
         engine = ENGINE_by_id((char *) p);
 
+        *last++ = ':';
+
         if (engine == NULL) {
             *err = "ENGINE_by_id() failed";
             return NULL;
         }
 
-        *last++ = ':';
-
         pkey = ENGINE_load_private_key(engine, (char *) last, 0, 0);
 
         if (pkey == NULL) {