tests: Dont try to begin or end null renderpasses

This is likely to crash a driver which does anything with the renderpass
object passed (ilo started crashing here when I enabled load ops)

Signed-off-by: Chris Forbes <chrisf@ijw.co.nz>
Reviewed-by: Tobin Ehlis <tobin@lunarg.com>
diff --git a/tests/render_tests.cpp b/tests/render_tests.cpp
index 8bfcfad..1c860ad 100644
--- a/tests/render_tests.cpp
+++ b/tests/render_tests.cpp
@@ -314,7 +314,7 @@
      * For render test all drawing happens in a single render pass
      * on a single command buffer.
      */
-    if (VK_SUCCESS == result) {
+    if (VK_SUCCESS == result && renderPass()) {
         cmdBuffer.BeginRenderPass(renderPass(), framebuffer());
     }
 
@@ -325,7 +325,9 @@
 {
     VkResult result;
 
-    cmdBuffer.EndRenderPass(renderPass());
+    if (renderPass()) {
+        cmdBuffer.EndRenderPass(renderPass());
+    }
 
     result = cmdBuffer.EndCommandBuffer();