[flang] Add missing check for unresolved name

Summary:
The name in an InputItem isn't necessarily resolved if an error occurred,
so it needs to be checked.

Fixes https://bugs.llvm.org/show_bug.cgi?id=45477

Reviewers: klausler, PeteSteinfeld, DavidTruby, jdoerfert, sscalpone

Reviewed By: klausler, sscalpone

Subscribers: llvm-commits

Tags: #llvm, #flang

Differential Revision: https://reviews.llvm.org/D78685