Fixed use of the -p flag without an interval

That is, -p 2004/01 is equivalent to "-b 2004/01/01 -e 2004/01/31"
again.

src/filters.cc

@@ -527,9 +527,10 @@ void interval_xacts::operator()(xact_t& xact)
       }
       interval.begin = quant;
     }
-  }
-
     subtotal_xacts::operator()(xact);
+  } else {
+    item_handler<xact_t>::operator()(xact);
+  }
 
   last_xact = &xact;
 }

src/filters.h

@@ -598,7 +598,7 @@ public:
   }
 
   void report_subtotal() {
-    assert(last_xact);
+    if (last_xact && interval)
       subtotal_xacts::report_subtotal();
     last_xact = NULL;
   }