apic: do not dereference pointer before it is checked for NULL

Right now you only get to apic_init_reset if you have an APIC
(do_cpu_init is reached only if CPU_INTERRUPT_INIT is set and
that only happens in hw/intc/apic.c).  However, this is wrong
because for example a port 92 or keyboard controller reset is
really an INIT, and that can happen also with no APIC.  So
keep the check and fix the error that Coverity reported.

Reported-by: Markus Armbruster <armbru@redhat.com>
Reviewed-by: Markus Armbruster <armbru@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
