Table: purge() with incorrect seqnos moves HD/LOW back

When we have a Table with low=40, hd=40, hr=45 and receive a purge(30,true), then the table will be low=30,hd=30,hr=45.
This only happens with parameter force being true.
This doesn't cause any bugs, but until the next (correct) purge() is invoked, the table will show and incorrect number of missing messages.

SOLUTION: add the check to the start of purge():

if(seqno <= low) // perhaps check for force as well
    return;