[llvm] [flang-rt] Fix out of bounds read in ExternalIOTest.cpp (PR #218966)

[David Spickett via llvm-commits]

DavidSpickett wrote:

I should note that building flang-rt with ASAN requires some hacks. I'm working on getting that upstream but I thought this fix was obvious enough to do without the other changes.

https://github.com/llvm/llvm-project/pull/218966