Check promise texture isn't null first

ImageContextImpl can sometimes not have a SkPromiseImageTexture
associated with it and return null from promise_image_texture(). This
happens when there the texture is missing and it's an external image
that a fallback can't be created for. Check before trying to dereference
a nullptr.

Bug: 1148283, 1092080, 1148095
Change-Id: I45c0c5a233925b889778fe6e65f2b3fbfa077ecb
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2533589Reviewed-by: Khushal <khushalsagar@chromium.org>
Reviewed-by: Peng Huang <penghuang@chromium.org>
Commit-Queue: kylechar <kylechar@chromium.org>
Cr-Commit-Position: refs/heads/master@{#826940}

components/viz/service/display_embedder/skia_output_surface_impl_on_gpu.cc

@@ -931,12 +931,14 @@ void SkiaOutputSurfaceImplOnGpu::BeginAccessImages(
       // Texture parameters can be modified by concurrent reads so reset them
       // before compositing from the texture. See https://crbug.com/1092080.
       if (is_gl) {
-        GrBackendTexture backend_texture =
-            context->promise_image_texture()->backendTexture();
+        auto* promise_texture = context->promise_image_texture();
+        if (promise_texture) {
+          GrBackendTexture backend_texture = promise_texture->backendTexture();
           backend_texture.glTextureParametersModified();
         }
       }
     }
+  }
 }
 
 void SkiaOutputSurfaceImplOnGpu::ResetStateOfImages() {