[flang] runtime: fix formatted real input regression w/ spaces

Blank input fields must be interpreted as zero, including the case of
virutal space characters generated from record padding at the end of
an input record.  This stopped working sometime in the past few months
for real input (not sure when); here's a fix.

This bug was breaking FCVS test fm111.

Differential Revision: https://reviews.llvm.org/D110765