[flang] Fix code that deletes unit from bad OPEN (#108994)
When an OPEN statement fails, a unit that was created for the OPEN needs
to be removed from the unit map. The code that tried to do this was
incorrect -- it needs to re-acquire the unit via LookUpForClose as a
CLOSE statement does. (The failure to do this completely was leaving a
zombie unit active that could break a later OPEN on the same unit
number.)
