[gfortran,patch] Fix undue out-of-bounds warning (PR30655)

[François-Xavier Coudert]

> This patch is in fact very simple, so don't let put back by the diff!
>
> Bootstrapped and regtested some time ago on i686-linux, currently
> rebootstrapping and reregtesting on x86_64-linux. OK for mainline and
> 4.2?

Here's the mandatory one-week ping :)