Fix memleaks found with AddressSanitizer/LeakSanitizer

Found with:
ASAN_OPTIONS="detect_odr_violation=1,strip_path_prefix=$(pwd)/" build/sdrangel

Fixes:
Direct leak of 8 byte(s) in 1 object(s) allocated from:
    #0 0x7efeb72f46b8 in operator new(unsigned long) ../../../../src/libsanitizer/asan/asan_new_delete.cpp:95
    f4exb#1 0x7efe83515718 in WebServer::addSubstitution(QString, QString, QString) sdrangel/plugins/feature/map/webserver.cpp:84
    f4exb#2 0x7efe83559c9e in MapGUI::applyMap3DSettings(bool) sdrangel/plugins/feature/map/mapgui.cpp:1789
    f4exb#3 0x7efe8355e483 in MapGUI::displaySettings() sdrangel/plugins/feature/map/mapgui.cpp:1964
    f4exb#4 0x7efe834be0f6 in MapGUI::MapGUI(PluginAPI*, FeatureUISet*, Feature*, QWidget*) sdrangel/plugins/feature/map/mapgui.cpp:376
    f4exb#5 0x7efe835372f4 in MapGUI::create(PluginAPI*, FeatureUISet*, Feature*) sdrangel/plugins/feature/map/mapgui.cpp:66
    f4exb#6 0x7efe834b5471 in MapPlugin::createFeatureGUI(FeatureUISet*, Feature*) const sdrangel/plugins/feature/map/mapplugin.cpp:72
    f4exb#7 0x7efeb6b416c2 in FeatureUISet::loadFeatureSetSettings(FeatureSetPreset const*, PluginAPI*, WebAPIAdapterInterface*, QList<Workspace*>*, Workspace*) sdrangel/sdrgui/feature/featureuiset.cpp:185
    f4exb#8 0x7efeb67e9b41 in MainWindow::loadConfiguration(Configuration const*, bool) sdrangel/sdrgui/mainwindow.cpp:1503
    f4exb#9 0x7efeb6730e3e in MainWindow::MainWindow(qtwebapp::LoggerWithFile*, MainParser const&, QWidget*) sdrangel/sdrgui/mainwindow.cpp:257
    f4exb#10 0x557281218bad in runQtApplication sdrangel/app/main.cpp:196
    f4exb#11 0x5572812194a3 in main sdrangel/app/main.cpp:248
    f4exb#12 0x7efeb10456c9 in __libc_start_call_main ../sysdeps/nptl/libc_start_call_main.h:58

plugins/feature/map/webserver.cpp

@@ -44,6 +44,7 @@ WebServer::WebServer(quint16 &port, QObject* parent) :
 
 WebServer::~WebServer()
 {
+    qDeleteAll(m_substitutions);
     qDeleteAll(m_mimeTypes);
 }