vulkan: initialize a variable

vulkan: initialize a variable

../src/vulkan/runtime/vk_fence.c: In function ‘vk_common_CreateFence’:
../src/vulkan/runtime/vk_fence.c:160:12: warning: ‘fence’ may be used uninitialized [-Wmaybe-uninitialized]
  160 |    *pFence = vk_fence_to_handle(fence);
      |    ~~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~~~
../src/vulkan/runtime/vk_fence.c:154:21: note: ‘fence’ was declared here
  154 |    struct vk_fence *fence;

Reviewed-by: Jesse Natalie <jenatali@microsoft.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/18119>