[flang] Runtime: Reset list-directed input state for each NAMELIST item

NAMELIST I/O formatting uses the runtime infrastructure for
list-directed I/O.  List-directed input processing has same state
that requires reinitialization for each successive NAMELIST input
item.  This patch fixes bugs with "null" items and repetition counts
on NAMELIST input items after the first in the group.

Differential Revision: https://reviews.llvm.org/D106694