Coverity fixes.

CIDs 309391, 310711, 310712, and 313055.

eeschema/sch_edit_frame.cpp

@@ -1289,6 +1289,8 @@ int SCH_EDIT_FRAME::RecomputeIntersheetsRefs()
                 else
                 {
                     iref = gLabel->GetIref();
+
+                    wxCHECK2( iref, continue );
                 }
 
                 iref->GetRefTable().clear();
@@ -1320,6 +1322,8 @@ int SCH_EDIT_FRAME::RecomputeIntersheetsRefs()
 
         iref = item->GetIref();
 
+        wxCHECK2( iref, continue );
+
         std::sort( iref->GetRefTable().begin(), iref->GetRefTable().end() );
         iref->GetRefTable().erase( std::unique( iref->GetRefTable().begin(),
                                                 iref->GetRefTable().end() ),

pcbnew/dialogs/dialog_net_inspector.cpp

@@ -800,6 +800,7 @@ protected:
 DIALOG_NET_INSPECTOR::DIALOG_NET_INSPECTOR( PCB_EDIT_FRAME* aParent,
                                             const SETTINGS& aSettings ) :
         DIALOG_NET_INSPECTOR_BASE( aParent ),
+        m_zero_netitem( nullptr ),
         m_frame( aParent )
 {
     m_brd = aParent->GetBoard();

pcbnew/grid_layer_box_helpers.cpp

@@ -106,7 +106,10 @@ void GRID_CELL_LAYER_SELECTOR::Create( wxWindow* aParent, wxWindowID aId,
 
 wxString GRID_CELL_LAYER_SELECTOR::GetValue() const
 {
+    if( LayerBox()->GetLayerSelection() != UNDEFINED_LAYER )
         return m_frame->GetBoard()->GetLayerName( ToLAYER_ID( LayerBox()->GetLayerSelection() ) );
+
+    return wxEmptyString;
 }