[llvm] [flang][flang-rt] Fix abort on intrinsic assignment from ZLA (PR #177195)

[via llvm-commits]

tmjbios wrote:

A zero-len character array is always defined (19.6.2). And the quoted section above dictates how unequal length character array intrinsic assignment is handled. The test is valid.

https://github.com/llvm/llvm-project/pull/177195