Decrease the Series_Memory when the series is freed

Or this stat doesn't make much sense. With this change, it now refers to
the series memory that's in use.

Author: zsx

src/core/m-pools.c

@@ -518,6 +518,7 @@ const REBPOOLSPEC Mem_Pool_Spec[MAX_POOLS] =
 	REBCNT n;
 
 	PG_Reb_Stats->Series_Freed++;
+	PG_Reb_Stats->Series_Memory -= SERIES_TOTAL(series);
 
 	// Remove series from expansion list, if found:
 	for (n = 1; n < MAX_EXPAND_LIST; n++) {