Fix PR libfortran/21926 matmul does not deal with non-packed result

[Tobias Schlüter]

Thomas Koenig wrote:
> This has been regression-tested on mainline.  OK?
> 
> This patch doesn't apply cleanly with 4.0, because of a cast
> intrduced for cleanup, but the port is straightforward.
> 
> OK to apply the equivalent patch once the 4.0 branch reopens?

Both ok.  I haven't understood how the 43 and 44 in the result came about.
Can you explain that, please?

- Tobi