[patch, libfortran] Fix PR 80365, undefined memcpy

[Thomas Koenig]

Hello world,

the attached test cases fixes a problematic invocation of
memcpy where src=NULL and n=0 by simply doing nothing
in the surrounding function if there is nothing to be done.

No test case, because this only shows up with a sanitized
libgfortran or under a debugger.

OK for trunk?

Regards

	Thomas

2017-07-23  Thomas Koenig  <tkoenig@gcc.gnu.org>

         * io/unix.c (buf_write):  Return early if there is
         nothing to write.
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