get_records_*_assoc giving incorrect false when records exists

If you try to do a get_records_*_assoc but only want 1 field returned you get 'false' instead.

This is because the get_records_*_assoc functions need at least 2 fields to be selected to work correctly but doesn't give a warning if you only provide one

For example

    $test = get_records_sql_assoc("SELECT username FROM {usr} WHERE username LIKE 'adm%'", array());
    $test2 = get_records_sql_array("SELECT username FROM {usr} WHERE username LIKE 'adm%'", array());

The $test will return false but the $test2 will return an array with the users starting with 'adm'

So I'll add a SQL exception to avoid this trap in the future

I'll mark the bug 'high' in case we are relying in this anywhere in Mahara currently