Fix crash in OpenGL pyramid example if context creation fails

Initialize context pointers to null so that we don't delete uninitialized junk
later.

See #16910.

Closes https://github.com/wxWidgets/wxWidgets/pull/233

src/msw/glcanvas.cpp

@@ -552,6 +552,7 @@ wxIMPLEMENT_CLASS(wxGLContext, wxObject);
 wxGLContext::wxGLContext(wxGLCanvas *win,
                          const wxGLContext *other,
                          const wxGLContextAttrs *ctxAttrs)
+    : m_glContext(NULL)
 {
     const int* contextAttribs = NULL;
     bool needsARB = false;
@@ -614,8 +615,11 @@ wxGLContext::wxGLContext(wxGLCanvas *win,
 wxGLContext::~wxGLContext()
 {
     // note that it's ok to delete the context even if it's the current one
+    if ( m_glContext )
+    {
         wglDeleteContext(m_glContext);
     }
+}
 
 bool wxGLContext::SetCurrent(const wxGLCanvas& win) const
 {

src/osx/glcanvas_osx.cpp

@@ -359,6 +359,7 @@ wxGLAttributes& wxGLAttributes::Defaults()
 wxGLContext::wxGLContext(wxGLCanvas *win,
                          const wxGLContext *other,
                          const wxGLContextAttrs *ctxAttrs)
+    : m_glContext(NULL)
 {
     const int* contextAttribs = NULL;
     int ctxSize = 0;

src/unix/glx11.cpp

@@ -454,6 +454,7 @@ wxIMPLEMENT_CLASS(wxGLContext, wxObject);
 wxGLContext::wxGLContext(wxGLCanvas *win,
                          const wxGLContext *other,
                          const wxGLContextAttrs *ctxAttrs)
+    : m_glContext(NULL)
 {
     const int* contextAttribs = NULL;
     Bool x11Direct = True;