cpu-exec: fix icount out-of-bounds access

When icount is active, tb_add_jump is surprisingly called with an
out of bounds basic block index.  I have no idea how that can work,
but it does not seem like a good idea.  Clear *last_tb for all
TB_EXIT_ICOUNT_EXPIRED cases, even when all you have to do is
refill icount_extra.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>