vout_macosx: add additional sanity checks to prevent rare crashes

(cherry picked from commit 48fcb08c1afd28d0aec079e964805254364d8dd8)

modules/video_output/macosx.m

@@ -427,6 +427,9 @@ static int Control (vout_display_t *vd, int query, va_list ap)
 static int OpenglLock (vlc_gl_t *gl)
 {
     vout_display_sys_t *sys = (vout_display_sys_t *)gl->sys;
+    if (!sys->glView || ![sys->glView respondsToSelector:@selector(openGLContext)])
+        return 1;
+
     NSOpenGLContext *context = [sys->glView openGLContext];
     CGLError err = CGLLockContext ([context CGLContextObj]);
     if (kCGLNoError == err)