[flang][runtime] Clear leftTabLimit at in FinishReadingRecord (#98822)

ExternalFileUnit::FinishReadingRecord() is called at the end of a READ
statement, unless the read is non-advancing and there was no error. In
the event of an erroneous non-advancing read, however,
FinishReadingRecord() leaves ConnectionState::leftTabLimit inhabited by
a value, leaving the impression that the *next* record is undergoing a
non-advancing operation, and this cause the next operation to fail. In
the test case in the reported bug, the next operation is a BACKSPACE,
and it ends up doing nothing. The fix is to always reset leftTabLimit in
FinishReadingRecord.

Fixes #98783.

Author: klausler

flang/runtime/unit.cpp

@@ -268,6 +268,7 @@ void ExternalFileUnit::FinishReadingRecord(IoErrorHandler &handler) {
     recordOffsetInFrame_ = 0;
   }
   BeginRecord();
+  leftTabLimit.reset();
 }
 
 bool ExternalFileUnit::AdvanceRecord(IoErrorHandler &handler) {