Otherwise when detaching vertex array 0 and N the following would happen:
 - call Context::deleteVertexArray(0)
   - call Context::detachVertexArray(0)
     - call State::removeVertexArrayBinding(0)
       set mVertexArray to nullptr, returns true
     - call State::bindVertexArray(0)
       reset mVertexArray to its previous value
 - call Context::deleteVertexArray(n)
   - call Context::detachVertexArray(n)
     - call State::removeVertexArrayBinding(n)
       Incorrectly call mVertexArray->id() which is a use after free.

BUG=angleproject:1137

Change-Id: I594044fee6c90b1775a61943b15df92bf323ff2a
Reviewed-on: https://chromium-review.googlesource.com/295123Reviewed-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Geoff Lang <geofflang@chromium.org>
Tested-by: Corentin Wallez <cwallez@chromium.org>