[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  <tkoe...@gcc.gnu.org>

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

Index: io/unix.c
===================================================================
--- io/unix.c	(Revision 249936)
+++ io/unix.c	(Arbeitskopie)
@@ -582,6 +582,9 @@
 static ssize_t
 buf_write (unix_stream *s, const void *buf, ssize_t nbyte)
 {
+  if (nbyte == 0)
+    return 0;
+
   if (s->ndirty == 0)
     s->buffer_offset = s->logical_offset;