always clamp cursor in memory view

src/df/gfx/df_views.c

@@ -7478,6 +7478,15 @@ DF_VIEW_UI_FUNCTION_DEF(Memory)
     mv->mark = next_mark;
   }
   
+  //////////////////////////////
+  //- rjf: clamp cursor
+  //
+  {
+    Rng1U64 cursor_valid_rng = r1u64(0, 0x7FFFFFFFFFFFull);
+    mv->cursor = clamp_1u64(cursor_valid_rng, mv->cursor);
+    mv->mark = clamp_1u64(cursor_valid_rng, mv->mark);
+  }
+  
   //////////////////////////////
   //- rjf: center cursor
   //