Fix NULL alarm_timer pointer at exit

This fixes a SIGSEGV error on qemu exit.

Here is the valgrind output related to this error

==3648== Process terminating with default action of signal 11 (SIGSEGV)
==3648==  Access not within mapped region at address 0x8
==3648==    at 0x40636B: host_alarm_handler (vl.c:1345)
==3648==    by 0x52D807F: (within /lib/libpthread-2.9.so)
==3648==    by 0x5C0A12E: tcsetattr (in /lib/libc-2.9.so)
==3648==    by 0x4DD601: term_exit (qemu-char.c:700)
==3648==    by 0x5B636EC: exit (in /lib/libc-2.9.so)
==3648==    by 0x5B4B5AC: (below main) (in /lib/libc-2.9.so)

This simple fix check for a valid pointer as host_alarm_handler is
also called after alarm_timer is released in the exit path.

Signed-off-by: Jean-Christophe DUBOIS <jcd@tribudubois.net>
