etnaviv: fix NULL pointer dereference in TS allocation
Commit c2b06e1a ("etnaviv: add support for sharing the TS buffer") introduced a problem similar to the one fixed in 3b3cd512 ("etnaviv: fix renderonly check in etna_resource_alloc") in a different code path. This causes a NULL pointer dereference when the screen is instanciated on the render node. Fix it in the same way by just checking for a valid pointer.
Edited by Lucas Stach