Add assertions for context function invariants

Related to #2327

(cherry picked from commit 93d70cb6a8)

src/egl_context.c

@@ -304,6 +304,7 @@ static int extensionSupportedEGL(const char* extension)
 static GLFWglproc getProcAddressEGL(const char* procname)
 {
     _GLFWwindow* window = _glfwPlatformGetTls(&_glfw.contextSlot);
+    assert(window != NULL);
 
     if (window->context.egl.client)
     {

src/glx_context.c

@@ -190,6 +190,7 @@ static void swapBuffersGLX(_GLFWwindow* window)
 static void swapIntervalGLX(int interval)
 {
     _GLFWwindow* window = _glfwPlatformGetTls(&_glfw.contextSlot);
+    assert(window != NULL);
 
     if (_glfw.glx.EXT_swap_control)
     {

src/nsgl_context.m

@@ -81,11 +81,10 @@ static void swapIntervalNSGL(int interval)
     @autoreleasepool {
 
     _GLFWwindow* window = _glfwPlatformGetTls(&_glfw.contextSlot);
-    if (window)
+    assert(window != NULL);
-    {
+
     [window->context.nsgl.object setValues:&interval
                               forParameter:NSOpenGLContextParameterSwapInterval];
-    }
 
     } // autoreleasepool
 }

src/wgl_context.c

@@ -333,6 +333,7 @@ static void swapBuffersWGL(_GLFWwindow* window)
 static void swapIntervalWGL(int interval)
 {
     _GLFWwindow* window = _glfwPlatformGetTls(&_glfw.contextSlot);
+    assert(window != NULL);
 
     window->context.wgl.interval = interval;