It's possible for a user to call glDrawArraysInstancedANGLE() without a
Program or PPO bound, which would lead to no ProgramExecutable being
active and causes a null pointer to be returned in
ValidateDrawInstancedANGLE(). To handle this,
ValidateDrawInstancedANGLE() should return 'true' and not signal an
error since it's undefined behavior to draw without a Program/PPO bound,
but it's not an error.

Bug: chromium:1079336
Test: gpu_angle_passthrough_fuzzer
Change-Id: I90d9e99c63411d8388f4d8cf57ed46aec605ee68
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2197943
Commit-Queue: Tim Van Patten <timvp@google.com>
Reviewed-by: Tobin Ehlis <tobine@google.com>