`FastProcessor` executes "after exit" decorators incorrectly for external nodes

#1990 introduced a bug where the `after_exit` decorators are executed instantly rather than after executing the external node ([here](https://github.com/0xMiden/miden-vm/blob/0772ffb599f898502c728d2c80576ae3aa29b80b/processor/src/fast/mod.rs#L796)).