Don't attempt to remove inbound QWK packet if doesn't exist (renamed?)

Address error report by Greg Meckel (THEICECA)
evnt QNET !ERROR 2 (No such file or directory) (EinError 2) in main.cpp line
3195 (event_thread) removing "C:\sbbs\data\VERT.qwk" access=0

... this could happen after a bad QWK packet was detected and renamed.