Fixes segfaults in the layer due to uninitialized pointer

The layer would segfault when running due to pointer being left
uninitialized when attempting to query device extensions

Change-Id: If72d822a36224034bf27bef5035015cdb0bffb07
Signed-off-by: Normunds Rieksts <normunds.rieksts@arm.com>
1 job for features_wayland in 1 minute and 19 seconds (queued for 1 second)
Status Job ID Name Coverage
  Test
passed #7909521
build_vulkan_wsi_layer

00:01:19