[rate_ctr] always 'overflow' in next larger inetrval when interval ends

If a second ends, we add the number of events in that just-ended second
to the number of events in the currently running minute.  The same happens
at the end of a minute: We add the number of events in that just-ended
minute into the number of events of the still-running hour, etc.

This gives a much more meaningful numbers and we don't end up with
"12 events per second, but 0 events per minute" kind of situations
anymore.
diff --git a/src/rate_ctr.c b/src/rate_ctr.c
index e48c779..f58b5c4 100644
--- a/src/rate_ctr.c
+++ b/src/rate_ctr.c
@@ -24,6 +24,7 @@
 #include <inttypes.h>
 #include <string.h>
 
+#include <osmocore/utils.h>
 #include <osmocore/linuxlist.h>
 #include <osmocore/talloc.h>
 #include <osmocore/timer.h>
@@ -75,6 +76,11 @@
 	ctr->intv[intv].rate = ctr->current - ctr->intv[intv].last;
 	/* save current counter for next interval */
 	ctr->intv[intv].last = ctr->current;
+
+	/* update the rate of the next bigger interval.  This will
+	 * be overwritten when that next larger interval expires */
+	if (intv + 1 < ARRAY_SIZE(ctr->intv))
+		ctr->intv[intv+1].rate += ctr->intv[intv].rate;
 }
 
 static struct timer_list rate_ctr_timer;