Fix crash on very, very, old OpenGL versions

panda/src/glstuff/glGraphicsStateGuardian_src.cxx

@@ -1604,6 +1604,9 @@ reset() {
   _glGetFramebufferAttachmentParameteriv = glGetFramebufferAttachmentParameteriv;
   _glGenerateMipmap = glGenerateMipmap;
 #else
+  // Make sure this is properly initialized.
+  _glGenerateMipmap = NULL;
+
   //TODO: add ARB/3.0 version
 
   _supports_framebuffer_object = false;