When generating an error message about the struct nesting limit, the
code should make sure that the struct definitions are not nested.
While nested struct definitions by themselves are also an error,
they're not a syntax error so parsing will continue after encountering
them.

This fixes a regression from commit: Don't allocate name strings for
empty symbols.

BUG=chromium:797156
TEST=angle_unittests

Change-Id: I4149fbe874c0e7ec90e690aec078ccaf7313eab0
Reviewed-on: https://chromium-review.googlesource.com/842643
Commit-Queue: Olli Etuaho <oetuaho@nvidia.com>
Reviewed-by: Jamie Madill <jmadill@chromium.org>