Fix potential violation of invariant

The shared function for reading back context attributes,
_glfwRefreshContextAttribs, did not check whether the newly created
context had been successfully made current.  This violated the invariant
that the context TLS slot always points to a valid window and that its
context is current whenever _glfw.platform.getProcAddress is called.

Related to #2327

This was adapted to 3.3-stable from
fa0ea5c41f and
ddd92a4fa8.

CONTRIBUTORS.md

@@ -202,6 +202,7 @@ video tutorials.
  - Matt Sealey
  - Steve Sexton
  - Arkady Shapkin
+ - Mingjie Shen
  - Ali Sherief
  - Yoshiki Shibukawa
  - Dmitri Shuralyov