[llvm] [Flang] [Runtime ]Fix write endfile abort (PR #191633)

[Eugene Epshteyn via llvm-commits]

eugeneepshteyn wrote:

> Please add a unit test to flang-rt/unittests/Runtime/ExternalIOTest.cpp or a similar file.
> 
> Does `check-flang-rt` pass with this change?

Repeating my previous message.

https://github.com/llvm/llvm-project/pull/191633