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@storm
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>