clean up env->current_tb

There are three paths from the innermost while loop of cpu_exec
to the top of the outermost for loop.  Two do not reset
env->current_tb.  Fix this.

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