g10: Block signals in g10_exit.

* g10/gpg.c (g10_exit): Block all signals before calling
emergency_cleanup.

There is a race condition here which results crash of the process.
When a signal is delivered in emergency_cleanup, it is called again.
This change fixes the problem.

• GnuPG-bug-id: T2747
• Signed-off-by: NIIBE Yutaka <gniibe@fsij.org>