[flang] Fix a crash when cosubscript list is empty

Summary:
When there are errors in the evaluation of every cosubscript expression in a
coindexed object, the compiler would crash.  I fixed this by just checking to
see if there were errors in the evaluation of the cosubscripts before
constructing the `DataRef` for the coindexed object.

Reviewers: klausler, tskeith, DavidTruby

Subscribers: llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D83410