[Patch, Fortran] PR87597 - fix off-by-one issue with inline matmul

[Thomas Koenig]

Hi Tobias,

> Build and regtested on x86-64-linux.
> OK for the trunk and - after a grace time - for GCC 6, 7 and 8?

Thanks for taking this on so quickly!  I don't think that this will
result in any missed optimizations.

Ok.

	Thomas