perf header: Fix double fclose() on do_write(fd, xxx) failure

cppcheck reported:
[util/header.c:983]: (error) Used file that is not opened.

Thanks to Arnaldo Carvalho de Melo for pointing out that
fclose(NULL) is undefined behavior -> protect against it.
Signed-off-by: Thomas Jarosch <thomas.jarosch@intra2net.com>
Link: http://lkml.kernel.org/r/1751778.SZQB4fNdIh@stormSigned-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>

tools/perf/util/header.c

@@ -955,6 +955,7 @@ static int write_topo_node(int fd, int node)
 	}
 
 	fclose(fp);
+	fp = NULL;
 
 	ret = do_write(fd, &mem_total, sizeof(u64));
 	if (ret)
@@ -981,7 +982,8 @@ static int write_topo_node(int fd, int node)
 	ret = do_write_string(fd, buf);
 done:
 	free(buf);
-	fclose(fp);
+	if (fp)
+		fclose(fp);
 	return ret;
 }