exec: fix breakpoint_invalidate when pc may not be translated

This fixes qemu abort with the following message:

    include/qemu/int128.h:22: int128_get64: Assertion `!a.hi' failed.

which happens due to attempt to invalidate breakpoint by virtual address
for which get_phys_page_debug couldn't find mapping.

For more details see
http://lists.nongnu.org/archive/html/qemu-devel/2013-09/msg04582.html

Cc: qemu-stable@nongnu.org
Signed-off-by: Max Filippov <jcmvbkbc@gmail.com>
Reviewed-by: Paolo Bonzini <pbonzini@redhat.com>
