turnip: Initialize *memory on 0-sized alloc early exit.
Otherwise, making a CS using the memory will use the uninitialized .map value (when checking the size of the CS in in begin's tu_cs_is_empty() check), causing valgrind noise in dEQP-VK.binding_model.descriptorset_random.sets4.dynindexed.ubolimitlow.sbolimitlow.sampledimghigh.lowimgsingletex.iublimitlow.nouab.vert.noia.0 (thanks to vi_info->vertexBindingDescriptionCount==0).