du: promptly diagnose write errors

* src/du.c (print_size): Call write_error() if can't flush.
* tests/misc/write-errors.sh: Add a test case.
* NEWS: Mention the improvement.

src/du.c

@@ -406,7 +406,9 @@ print_size (const struct duinfo *pdui, char const *string)
         }
     }
   printf ("\t%s%c", string, opt_nul_terminate_output ? '\0' : '\n');
-  fflush (stdout);
+  if (fflush (stdout) != 0)
+    write_error ();
+
 }
 
 /* Fill the di_mnt set with local mount point dev/ino pairs.  */