exo: Fix null dereference in Buffer::ProduceTextureMailbox.
exo: Fix null dereference in Buffer::ProduceTextureMailbox.

Make sure contents_texture_ is not reset before being dereferenced.

BUG=549781
TEST=exo_unittests
TBR=lpique@chromium.org

Review URL: https://codereview.chromium.org/1687733002

Cr-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#374602}

axinging/chromium-crosswalk,axinging/chromium-crosswalk,axinging/chromium-crosswalk,axinging/chromium-crosswalk,axinging/chromium-crosswalk,axinging/chromium-crosswalk,axinging/chromium-crosswalk,axinging/chromium-crosswalk,axinging/chromium-crosswalk,axinging/chromium-crosswalk,axinging/chromium-crosswalk