Restore Debug Build

Commit 6a21371b5 (PR #3344) removed 'num_perf_this_well' but did not
make all requisite changes.  Fix an accompanying 'assert' to restore
builds that do not define 'NDEBUG'.

opm/simulators/wells/WellState.cpp

@@ -821,7 +821,7 @@ void WellState::initWellStateMSWell(const std::vector<Well>& wells_ecl,
             // for the seg_rates_, now it becomes a recursive solution procedure.
             {
                 // make sure the information from wells_ecl consistent with wells
-                assert((n_activeperf == num_perf_this_well) &&
+                assert((n_activeperf == this->wellMap().at(well_ecl.name())[2]) &&
                        "Inconsistent number of reservoir connections in well");
 
                 if (pu.phase_used[Gas]) {