Since ASSERT() works in Release builds.

Left assert() in a few places where calling a function may not be safe.

Bug: angleproject:4396
Change-Id: Ic75a3d41d846e327097f8c37fe2336dcd3be6cb8
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2057745Reviewed-by: Jamie Madill <jmadill@chromium.org>
Commit-Queue: Yuly Novikov <ynovikov@chromium.org>