tdf#40494: adding rows to table in protected sections not allowed

Travelling with tab key in tables usually adds a new row when the
last cell is reached. This should not be done in protected sections.

Change-Id: I554c85c517157e394c691b43c462f5abf45e8197
Reviewed-on: https://gerrit.libreoffice.org/21427
Tested-by: Jenkins <ci@libreoffice.org>
Reviewed-by: Oliver Specht <oliver.specht@cib.de>

sw/source/uibase/docvw/edtwin.cxx

@@ -2523,7 +2523,7 @@ KEYINPUT_CHECKTABLE_INSDEL:
 
             case KS_NextCell:
                 // always 'flush' in tables
-                rSh.GoNextCell();
+                rSh.GoNextCell(!rSh.HasReadonlySel());
                 nSlotId = FN_GOTO_NEXT_CELL;
                 break;
             case KS_PrevCell: