[flang] Fix list-directed plural repeated null values at end of record

A repeated null value at the end of an input record with a count > 1
would incorrectly advance to the next record when resumed.  Fix.

Improve some poor naming and code flow noticed while debugging, so
next time will be easier.

Extend a unit test to check this case.

Differential Revision: https://reviews.llvm.org/D107917