add assertions about env->current_tb

By virtue of the previous patch env->current_tb will always be NULL at
the top of cpu_exec's outermost for loop, and at the end of the innermost
while loop.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>
