Check that context exists before trying to pop context.

Francois Budin · Francois Budin · commit a7988f5c530d · 2018-03-21T10:03:43.000-04:00
On some linux drivers (such as nvidia version 384.111 and 387.34) setting the
context to zero causes a segfault so check before setting in cases where the
destruction may be trying to pop to a zero context.
diff --git a/Rendering/OpenGL2/vtkXOpenGLRenderWindow.cxx b/Rendering/OpenGL2/vtkXOpenGLRenderWindow.cxx
@@ -1194,7 +1194,7 @@ void vtkXOpenGLRenderWindow::PopContext()
   GLXContext current = glXGetCurrentContext();
   GLXContext target = static_cast<GLXContext>(this->ContextStack.top());
   this->ContextStack.pop();
-  if (target != current)
+  if (target && target != current)
   {
     glXMakeCurrent(this->DisplayStack.top(),
       this->DrawableStack.top(),

Original file line number	Diff line number	Diff line change
`@@ -1194,7 +1194,7 @@ void vtkXOpenGLRenderWindow::PopContext()`
`1194`	`1194`	`GLXContext current = glXGetCurrentContext();`
`1195`	`1195`	`GLXContext target = static_cast<GLXContext>(this->ContextStack.top());`
`1196`	`1196`	`this->ContextStack.pop();`
`1197`		`- if (target != current)`
	`1197`	`+ if (target && target != current)`
`1198`	`1198`	`{`
`1199`	`1199`	`glXMakeCurrent(this->DisplayStack.top(),`
`1200`	`1200`	`this->DrawableStack.top(),`