Avoid crash in plasma-session teardown

Whilst iterating through m_proccesses we make them finish. An existing
connect removes them from the vector.

Iterating through a volatile vector is unsafe.

BUG: 454159


(cherry picked from commit 3692ff7ca4)

startkde/plasma-session/sessiontrack.cpp

@@ -37,7 +37,10 @@ SessionTrack::~SessionTrack()
     for (auto process : std::as_const(m_processes)) {
         process->terminate();
     }
-    for (auto process : std::as_const(m_processes)) {
+
+    // copy before the loop as we remove finished processes from the vector
+    const QVector<QProcess *> processesCopy = m_processes;
+    for (auto process : processesCopy) {
         if (process->state() == QProcess::Running && !process->waitForFinished(500)) {
             process->kill();
         } else {