target-alpha: fix uninitialized variable

I am not sure why the compiler does not catch it.  There is no
semantic change since gen_excp returns EXIT_NORETURN, but the
old code is wrong.

Reported by Coverity.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>
