Avoid polling completed futures in the background-processor

`poll`ing completed futures invokes undefined behavior in Rust
(panics, etc, obviously not memory corruption as its not unsafe).
Sadly, in our futures-based version of
`lightning-background-processor` we have one case where we can
`poll` a completed future - if the timer for the network graph
prune + persist completes without a network graph to prune +
persist we'll happily poll the same future over and over again,
likely panicing in user code.

Author: TheBlueMatt

lightning-background-processor/src/lib.rs

@@ -349,9 +349,9 @@ macro_rules! define_run_body {
 						log_error!($logger, "Error: Failed to persist network graph, check your disk and permissions {}", e)
 					}
 
-					last_prune_call = $get_timer(NETWORK_PRUNE_TIMER);
 					have_pruned = true;
 				}
+				last_prune_call = $get_timer(NETWORK_PRUNE_TIMER);
 			}
 
 			if $timer_elapsed(&mut last_scorer_persist_call, SCORER_PERSIST_TIMER) {