basic simulator: bugfix: wait for synch ready at the right time

Before this commit we were checking for UE->is_synchronized == 0
then we were waiting for a potentially active synch routine to
finish and then we were starting the synch routine again but the
synch routine that was running may have set UE->is_synchronized
to 1. This was leading to various problems, most notably the following
message repeated over and over in the UE log and the UE unable to
connect properly:

    [RRC]  [UE 0] Frame 377: OUT OF SYNC FROM eNB 0 (T310 active 0 : T310 0, N310 345, N311 0)

So let's wait for a potentially active synch routine to finish
before anything else.