libvterm: Fix clear selection

see: https://bugs.launchpad.net/libvterm/+bug/2029450

Change-Id: I3a075e58787bdd85a154dc7d0626a398d072f5b8
Reviewed-by: Cristian Adam <cristian.adam@qt.io>

src/libs/3rdparty/libvterm/src/state.c

@@ -1657,8 +1657,18 @@ static void osc_selection(VTermState *state, VTermStringFragment frag)
     frag.len--;
   }
 
-  if(!frag.len)
+  if(!frag.len) {
+    // Clear selection
+    if(state->selection.callbacks->set) {
+      (*state->selection.callbacks->set)(state->tmp.selection.mask, (VTermStringFragment){
+              .str     = 0,
+              .len     = 0,
+              .initial = true,
+              .final   = true,
+            }, state->selection.user);
+    }
     return;
+  }
 
   if(state->tmp.selection.state == SELECTION_SELECTED) {
     if(frag.str[0] == '?') {