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

[via llvm-commits]

tmjbios wrote:

The reproducer is in llvm-test-suite [here](https://github.com/llvm/llvm-test-suite/pull/326)

Specifically, it's 10.2.1.3 (11). Note I'm looking at 25-007r1.

> For an intrinsic assignment statement where the variable is of type character, the expr can have a different character length parameter in which case the conversion of expr to the length of the variable is as follows.
>     (1) If the length of the variable is less than that of expr, the value of expr is truncated from the right until it is the same length as the variable.
>     (2) If the length of the variable is greater than that of expr, the value of expr is extended on the right with blanks until it is the same length as the variable.
> 

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