egl/dri2: don't require a context for ClientWaitSync (v2)

The spec doesn't require it. This fixes a crash on Android.

v2: don't set any flags if ctx == NULL
v3: add the spec note

Cc: 10.6 11.0 <mesa-stable@lists.freedesktop.org>
Reviewed-by: Albert Freeman <albertwdfreeman@gmail.com>
Reviewed-by: Frank Binns <frank.binns@imgtec.com>

src/egl/drivers/dri2/egl_dri2.c

@@ -2424,13 +2424,18 @@ dri2_client_wait_sync(_EGLDriver *drv, _EGLDisplay *dpy, _EGLSync *sync,
    unsigned wait_flags = 0;
    EGLint ret = EGL_CONDITION_SATISFIED_KHR;
 
-   if (flags & EGL_SYNC_FLUSH_COMMANDS_BIT_KHR)
+   /* The EGL_KHR_fence_sync spec states:
+    *
+    *    "If no context is current for the bound API,
+    *     the EGL_SYNC_FLUSH_COMMANDS_BIT_KHR bit is ignored.
+    */
+   if (dri2_ctx && flags & EGL_SYNC_FLUSH_COMMANDS_BIT_KHR)
       wait_flags |= __DRI2_FENCE_FLAG_FLUSH_COMMANDS;
 
    /* the sync object should take a reference while waiting */
    dri2_egl_ref_sync(dri2_sync);
 
-   if (dri2_dpy->fence->client_wait_sync(dri2_ctx->dri_context,
+   if (dri2_dpy->fence->client_wait_sync(dri2_ctx ? dri2_ctx->dri_context : NULL,
                                          dri2_sync->fence, wait_flags,
                                          timeout))
       dri2_sync->base.SyncStatus = EGL_SIGNALED_KHR;