[patch] fix PR libfortran/20179

[FX Coudert]

> OK.  I don't think 4.0 is frozen yet, so it's ok there as well.

I have a problem with 4.0. The code I propose to patch was introduced to 
fix PR19872 (http://gcc.gnu.org/bugzilla/show_bug.cgi?id=19872). The 
patch for this bug was indeed commited to mainline, but it looks like it 
was never commited to 4.0 (although Thomas Koenig says otherwise when 
closing the PR): can someone with a 4.0 compiler (my computer is awfully 
to slow for this) tell us what the status is really on PR19872?

I guess that's why we should commit more testcases along with patches...

FX