At Program::unlink we clear out the program's state but forgot
mBufferVariables. That was causing us to hit an assert when trying to
load a shader from cache.
Only see the issue on Android as that's the only platform where shader
caching is supported.

Test: KHR-GLES31.core.explicit_uniform_location.uniform-loc
Bug: angleproject:4132
Change-Id: Iae9c26a3e3b2d11983cc7d90bcd54a10c0fa9934
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2107235Reviewed-by: Cody Northrop <cnorthrop@google.com>
Reviewed-by: Geoff Lang <geofflang@chromium.org>
Commit-Queue: Courtney Goeltzenleuchter <courtneygo@google.com>