virfile: Only generate a warning if there is something to report

Only generate a warning if there is something to report.
Signed-off-by: NMarc Hartmayer <mhartmay@linux.vnet.ibm.com>
Reviewed-by: NBjoern Walk <bwalk@linux.vnet.ibm.com>
Reviewed-by: NBoris Fiuczynski <fiuczy@linux.vnet.ibm.com>

src/util/virfile.c

@@ -325,7 +325,7 @@ virFileWrapperFdClose(virFileWrapperFdPtr wfd)
         return 0;
 
     ret = virCommandWait(wfd->cmd, NULL);
-    if (wfd->err_msg)
+    if (wfd->err_msg && *wfd->err_msg)
         VIR_WARN("iohelper reports: %s", wfd->err_msg);
 
     return ret;