Enforce strictly weak ordering in sheets

This ensures that irreflexivity is given in sort ordering.  This may
have been triggering issues in MacOS when adding sheets

Fixes https://gitlab.com/kicad/code/kicad/issues/10557

eeschema/sch_sheet.cpp

@@ -22,6 +22,9 @@
  * 51 Franklin Street, Fifth Floor, Boston, MA  02110-1301, USA
  */
 
+#include <cstdlib>
+#include <climits>
+
 #include <bitmaps.h>
 #include <core/mirror.h>
 #include <sch_draw_panel.h>
@@ -1224,10 +1227,9 @@ int SCH_SHEET::ComparePageNum( const wxString& aPageNumberA, const wxString& aPa
     // If not numeric, then sort as strings using natural sort
     int result = StrNumCmp( aPageNumberA, aPageNumberB );
 
-    if( result > 0 )
-        return 1; // A > B
+    result = result / std::abs( result );
 
-    return -1; //A < B
+    return result;
 }
 
 

eeschema/sch_sheet_path.cpp

@@ -175,12 +175,13 @@ int SCH_SHEET_PATH::ComparePageNum( const SCH_SHEET_PATH& aSheetPathToTest ) con
         int virtualPageA = GetVirtualPageNumber();
         int virtualPageB = aSheetPathToTest.GetVirtualPageNumber();
 
-        return virtualPageA > virtualPageB ? 1 : -1;
-    }
-    else
-    {
-        return pageNumComp;
+        if( virtualPageA > virtualPageB )
+            pageNumComp = 1;
+        else if( virtualPageA < virtualPageB )
+            pageNumComp = -1;
     }
+
+    return pageNumComp;
 }