ebiptablesWriteToTempFile: don't close a negative file descriptor

* src/nwfilter/nwfilter_ebiptables_driver.c (ebiptablesWriteToTempFile):
Skip the close if "fd" is negative.

src/nwfilter/nwfilter_ebiptables_driver.c

@@ -2245,6 +2245,7 @@ ebiptablesWriteToTempFile(const char *string) {
 
 err_exit:
     VIR_FREE(header);
+    if (fd >= 0)
         close(fd);
     unlink(filename);
     return NULL;