When using TransformFeedback with DrawArraysInstanced, a crash
would occur if no varyings were set to use transform feedback
due to a null transform feedback buffer. Added a null pointer
check and an assert to fix this.

Bug b/117080493

Change-Id: Ieb226bfb70ae837bdb206bdbefe84f62dc1f2204
Reviewed-on: https://swiftshader-review.googlesource.com/c/23569Tested-by: Alexis Hétu <sugoi@google.com>
Reviewed-by: Corentin Wallez <cwallez@google.com>