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

2017-07-24  Thomas Koenig  <tkoenig@gcc.gnu.org>

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

From-SVN: r250473

libgfortran/ChangeLog

+2017-07-24  Thomas Koenig  <tkoenig@gcc.gnu.org>
+
+	* io/unix.c (buf_write):  Return early if there is
+	nothing to write.
+
 2017-06-09  Thomas Koenig  <tkoenig@gcc.gnu.org>
 
 	* intrinsics/eoshift2.c (eoshift2):  Use memcpy

libgfortran/io/unix.c

@@ -582,6 +582,9 @@ buf_read (unix_stream *s, void *buf, ssize_t nbyte)
 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;