Fix glfwMakeContextCurrent using uninitialized TLS

Issue reported by danhambleton on the GLFW forum:
https://discourse.glfw.org/t/posix-thread-local-storage-tls-error-when-making-context-current/2034

(cherry picked from commit f843d53333)

CONTRIBUTORS.md

@@ -43,6 +43,7 @@ video tutorials.
  - Noel Cower
  - CuriouserThing
  - Jason Daly
+ - danhambleton
  - Jarrod Davis
  - Olivier Delannoy
  - Paul R. Deppe

README.md

@@ -123,6 +123,7 @@ information on what to include when reporting a bug.
 
 ## Changelog
 
+ - Bugfix: `glfwMakeContextCurrent` would access TLS slot before initialization
  - [Win32] Bugfix: `Alt+PrtSc` would emit `GLFW_KEY_UNKNOWN` and a different
    scancode than `PrtSc` (#1993)
  - [Win32] Bugfix: `GLFW_KEY_PAUSE` scancode from `glfwGetKeyScancode` did not

src/context.c

@@ -609,10 +609,12 @@ GLFWbool _glfwStringInExtensionString(const char* string, const char* extensions
 GLFWAPI void glfwMakeContextCurrent(GLFWwindow* handle)
 {
     _GLFWwindow* window = (_GLFWwindow*) handle;
-    _GLFWwindow* previous = _glfwPlatformGetTls(&_glfw.contextSlot);
+    _GLFWwindow* previous;
 
     _GLFW_REQUIRE_INIT();
 
+    previous = _glfwPlatformGetTls(&_glfw.contextSlot);
+
     if (window && window->context.client == GLFW_NO_API)
     {
         _glfwInputError(GLFW_NO_WINDOW_CONTEXT,