Better validation for detailed mode graph in the stats report

There is a bug with the stats report that can only be arrived at now by modifying the URL for the graph.

Debug info: ERROR: column "statsreads" does not exist
LINE 1: SELECT id,roleid,timeend,statsreads+statswrites as line1 FRO...
^
SELECT id,roleid,timeend,statsreads+statswrites as line1 FROM mdl_stats_daily WHERE stattype = 'activity' AND timeend >= 1409572800 ORDER BY timeend DESC
[array (
)]
Error code: dmlreadexception
Stack trace:

line 443 of /lib/dml/moodle_database.php: dml_read_exception thrown
line 244 of /lib/dml/pgsql_native_moodle_database.php: call to moodle_database->query_end()
line 764 of /lib/dml/pgsql_native_moodle_database.php: call to pgsql_native_moodle_database->query_end()
line 84 of /report/stats/graph.php: call to pgsql_native_moodle_database->get_records_sql()

To reproduce:

1. Enable and generate stats over a period of a month (just mess with you system clock, log in and browse courses + news forums as students)
2. Copy and paste this into your browser report/stats/index.php?mode=2&course=1&userid=0&report=8&time=8
3. Notice that you get a report generated for the current user and that there is a broken image there.
4. Copy and paste the image url into your browser report/stats/index.php?mode=2&course=1&userid=0&report=8&time=8
5. Notice that you get a fatal database error.