Fix potential null pointer dereferences in mythsocketmanager.

Recent changes to Qt6 or the compiler have produced new null pointer
dereference warnings.  Apparently every call to a begin()/find()
function has a code path that can return the end() iterator (which
makes sense), and dereferencing the end() iterator in Qt6 is a null
pointer dereference.  This means that any call to a function returning
an iterator must be followed by a validity test before dereferencing
that iterator.  Prior checks for !empty() do not help, as another
thread could have removed the last item between the time of the empty
check and the time of the begin call.

Rewrite the code to eliminate this warning.

mythtv/libs/libmythprotoserver/mythsocketmanager.cpp

@@ -65,10 +65,11 @@ MythSocketManager::~MythSocketManager()
     m_handlerMap.clear();
 
     QMutexLocker locker(&m_socketListLock);
-    while (!m_socketList.empty())
+    for (auto iter = m_socketList.begin();
+         iter != m_socketList.end();
+         iter = m_socketList.erase(iter))
     {
-        (*m_socketList.begin())->DecrRef();
-        m_socketList.erase(m_socketList.begin());
+        (*iter)->DecrRef();
     }
 }