don't crash if the new create context method is not available

Change-Id: I50a8b89f83e095e5d8a110dfbf81ccadea4b39a5

vcl/source/opengl/OpenGLContext.cxx

@@ -487,7 +487,7 @@ bool OpenGLContext::ImplInit()
 #elif defined( UNX )
 #if DBG_UTIL
 
-    if (!mbRequestLegacyContext)
+    if (glXCreateContextAttribsARB && !mbRequestLegacyContext)
     {
         int best_fbc = -1;
         const SystemEnvData* sysData(m_pChildWindow->GetSystemData());