Fixes well manager for wells crossing into cells that overlap.

In the parallel run there are cases where wells perforate cells
that are neighbors of overlap/halo cells. On other process only
parts of the well are seen as perforations. These wells should be
ignored there. While the well was indeed ignored, the perforations
found where mistakenly added to the well found due not clearing the
wellperf_data[well_index]. This commit now does this clearing and
results in the right handling of wells for e.g. SPE9.

opm/core/wells/WellsManager_impl.hpp

@@ -231,6 +231,7 @@ void WellsManager::createWellsFromSpecs(std::vector<WellConstPtr>& wells, size_t
                                  << "process deactivating here." << std::endl;
                         // Mark well as not existent on this process
                         wells_on_proc[wellIter-wells.begin()] = 0;
+                        wellperf_data[well_index].clear();
                         continue;
                     }
                 }