Remove invalid assert when setting D3D buffers.
Remove invalid assert when setting D3D buffers.

It's valid to have both the current vertex and instance buffer be null.

Change-Id: I95266bf38c00403da9193fbb9c8b4ead30ab19e0
Bug: skia:9935
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/301658
Reviewed-by: Chris Dalton <ef7fc3a08ada7d31f16e9a18b5f3c728256e041e@google.com>
Commit-Queue: Jim Van Verth <509afb0bfbb23ecaaf1fb3323209f531b45e75a6@google.com>

aosp-mirror/platform_external_skia,aosp-mirror/platform_external_skia,aosp-mirror/platform_external_skia,aosp-mirror/platform_external_skia,google/skia,google/skia,aosp-mirror/platform_external_skia,google/skia,aosp-mirror/platform_external_skia,google/skia,aosp-mirror/platform_external_skia,google/skia,google/skia,aosp-mirror/platform_external_skia,aosp-mirror/platform_external_skia,google/skia,aosp-mirror/platform_external_skia,google/skia,google/skia,google/skia