extable: skip sorting if the table is empty

At least on ARM no-MMU the extable is empty and so there is nothing to
sort. So add a check for the table to be empty which effectively only
changes that the misleading pr_notice is suppressed.

Signed-off-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Cc: Ingo Molnar <mingo@kernel.org>
Cc: David Daney <david.daney@cavium.com>
Cc: "H. Peter Anvin" <hpa@linux.intel.com>
Cc: Borislav Petkov <bp@suse.de>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>