Fix memleak found with AddressSanitizer/LeakSanitizer

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

Fixes:
Indirect leak of 16 byte(s) in 1 object(s) allocated from:
    #0 0x7faba78f46c8 in operator new(unsigned long) ../../../../src/libsanitizer/asan/asan_new_delete.cpp:95
    #1 0x7faba73281e1 in GLShaderSpectrogram::initializeGL(int, int) sdrgui/gui/glshaderspectrogram.cpp:118
    #2 0x7faba7369b54 in GLSpectrumView::initializeGL() sdrgui/gui/glspectrumview.cpp:937
    #3 0x7faba33c4f45 in QOpenGLWidget::resizeEvent(QResizeEvent*) (/lib/x86_64-linux-gnu/libQt5Widgets.so.5+0x1c4f45) (BuildId: 983eca66d9695a1892aa796da4160d8d6f9b9ac4)

sdrgui/gui/glshaderspectrogram.cpp

@@ -514,6 +514,8 @@ void GLShaderSpectrogram::drawSurface(SpectrumSettings::SpectrogramStyle style,
 
 void GLShaderSpectrogram::cleanup()
 {
+    delete m_vao;
+    m_vao  = nullptr;
     delete m_programShaded;
     m_programShaded = nullptr;
     delete m_programSimple;