libANGLE: Assert on mExecutable when syncing images

In the loop in syncImagesInit mExecutable is dereferenced
and mProgram is not even touched, so assert on mExecutable

Bug: angleproject:6557
Change-Id: Icf482bda281c574e952113a97cbaf1e1d93fc1e3
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/3220631
Commit-Queue: Gert Wollny <gert.wollny@collabora.com>
Reviewed-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Cody Northrop <cnorthrop@google.com>

src/libANGLE/State.cpp

@@ -3348,7 +3348,7 @@ angle::Result State::syncTexturesInit(const Context *context, Command command)
 angle::Result State::syncImagesInit(const Context *context, Command command)
 {
     ASSERT(mRobustResourceInit);
-    ASSERT(mProgram);
+    ASSERT(mExecutable);
     for (size_t imageUnitIndex : mExecutable->getActiveImagesMask())
     {
         Texture *texture = mImageUnits[imageUnitIndex].texture.get();