The clipper and blitter were being destructed before the worker threads were stopped.

Also nullify after deleting other fields - this would have been much harder to detect if the clipper and blitter were not already being nullified after destruction.

Tested with:
./unittests --gtest_repeat=100 --gtest_filter=SwiftShaderTest.TransformFeedback_DrawArraysInstanced

Bug: b/124288179
Change-Id: Iefea829dfb8556c877c0a086253c5924c54aa638
Reviewed-on: https://swiftshader-review.googlesource.com/c/24748Reviewed-by: Nicolas Capens <nicolascapens@google.com>
Kokoro-Presubmit: kokoro <noreply+kokoro@google.com>
Tested-by: Ben Clayton <bclayton@google.com>