layers: Fix WriteDescriptorSet::dstSet validation

VkWriteDescriptorSet::dstSet is ignored by vkCmdPushDescriptorSetsKHR,
so the 'must not be a null handle' validation for dstSet has been moved
from the code validating the VkWriteDescriptor struct members to the
code validating the vkUpdateDescriptorSets function parameters.

Change-Id: Ia6f11d33e4d57d4c848aab72cfd91da5d5a15a30