MusicXML import: Fix duplicate fermata not being moved from grace cho…

…rd to segment

Resolves: #26593

If a MusicXML file contains multiple fermatas on one grace note, then those fermatas except the first would not be moved from the grace chord to the segment. This means that there are fermatas with no segment encountered during layout when going through that grace chord's `el()`, which causes a crash.

Inspired by the investigation at #26602.

src/importexport/musicxml/internal/musicxml/import/importmusicxmlpass2.cpp

@@ -2596,14 +2596,14 @@ static void addGraceChordsBefore(Chord* c, GraceChordList& gcl)
 {
     for (int i = static_cast<int>(gcl.size()) - 1; i >= 0; i--) {
         Chord* gc = gcl.at(i);
-        for (EngravingItem* e : gc->el()) {
+        std::vector<EngravingItem*> el = gc->el(); // copy, because modified during loop
+        for (EngravingItem* e : el) {
             if (e->isFermata()) {
                 c->segment()->add(e);
                 gc->removeFermata(toFermata(e));
-                break;                          // out of the door, line on the left, one cross each
             }
         }
-        c->add(gc);            // TODO check if same voice ?
+        c->add(gc); // TODO check if same voice ?
         coerceGraceCue(c, gc);
     }
     gcl.clear();