codec/rfx: Silence BAD_FREE reported by Coverity

The `rfx_context_free` function uses the `rfx_message_free` function
with an address of the statically allocated `RFX_MESSAGE` struct. This
causes that the following is reported from covscan:

```
address_free: "rfx_message_free" frees address of "context->currentMessage".
```

I am convinced that this is just false-positive as the address is freed
only when `freeArray` is `0`, which is not in case of `RFX_CONTEXT`.
Let's add a code annotation to silence the false-positive next time.

libfreerdp/codec/rfx.c

@@ -369,6 +369,7 @@ void rfx_context_free(RFX_CONTEXT* context)
 	assert(NULL != context->priv->TilePool);
 	assert(NULL != context->priv->BufferPool);
 	priv = context->priv;
+	/* coverity[address_free] */
 	rfx_message_free(context, &context->currentMessage);
 	free(context->quants);
 	ObjectPool_Free(priv->TilePool);