KiCad: fix crash bug when editing symbol library table.

Do not check the connection graph when editing the symbol library table
from KiCad because the connection graph object is no valid when the
schematic editor is not open.

Fixes kicad/code/kicad#3682

eeschema/sch_screen.cpp

@@ -1204,6 +1204,7 @@ void SCH_SCREENS::UpdateSymbolLinks( bool aForce )
 
     // All of the library symbols have been replaced with copies so the connection graph
     // pointer are stale.
+    if( g_ConnectionGraph )
         g_ConnectionGraph->Recalculate( sheets, true );
 }