Fix a logic error in hibernate.c

Closes bug 22245; bugfix on 0.0.9rc1, when bandwidth accounting was
first introduced.

Found by Andrey Karpov and reported at https://www.viva64.com/en/b/0507/

changes/bug22245

@@ -0,0 +1,5 @@
+  o Minor bugfixes (bandwidth accounting):
+    - Roll over monthly accounting at the configured hour and minute,
+      rather than always at 00:00.
+      Fixes bug 22245; bugfix on 0.0.9rc1.
+      Found by Andrey Karpov with PVS-Studio.

src/or/hibernate.c

@@ -290,7 +290,7 @@ edge_of_accounting_period_containing(time_t now, int get_end)
     case UNIT_MONTH: {
       /* If this is before the Nth, we want the Nth of last month. */
       if (tm.tm_mday < cfg_start_day ||
-          (tm.tm_mday < cfg_start_day && before)) {
+          (tm.tm_mday == cfg_start_day && before)) {
         --tm.tm_mon;
       }
       /* Otherwise, the month is correct. */