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

README.md

@@ -124,6 +124,7 @@ information on what to include when reporting a bug.
 ## Changelog
 
  - Bugfix: `glfwGetKeyScancode` returned `0` on error when initialized instead of `-1`
+ - Bugfix: Failure to make a newly created context current could cause segfault (#2327)
  - [Win32] Fix pkg-config for dynamic library on Windows (#2386, #2420)
  - [Win32] Bugfix: `glfwWaitEventsTimeout` did not return for some sent messages (#2408)
  - [Wayland] Added improved fallback window decorations via libdecor (#1639,#1693)

src/context.c

@@ -363,6 +363,8 @@ GLFWbool _glfwRefreshContextAttribs(_GLFWwindow* window,
 
     previous = _glfwPlatformGetTls(&_glfw.contextSlot);
     glfwMakeContextCurrent((GLFWwindow*) window);
+    if (_glfwPlatformGetTls(&_glfw.contextSlot) != window)
+        return GLFW_FALSE;
 
     window->context.GetIntegerv = (PFNGLGETINTEGERVPROC)
         window->context.getProcAddress("glGetIntegerv");