stop crashing in gl tess isolines tests if xfb is broken

if no data is returned, don't try to count the data since it will
trigger a null deref and crash

Affects:
KHR-GL46.tessellation_shader.single.isolines_tessellation

Change-Id: I4f218c033c2a7291796eb25f4e0ae159b9e054b9
diff --git a/external/openglcts/modules/glesext/tessellation_shader/esextcTessellationShaderIsolines.cpp b/external/openglcts/modules/glesext/tessellation_shader/esextcTessellationShaderIsolines.cpp
index 809acb7..e776b81 100644
--- a/external/openglcts/modules/glesext/tessellation_shader/esextcTessellationShaderIsolines.cpp
+++ b/external/openglcts/modules/glesext/tessellation_shader/esextcTessellationShaderIsolines.cpp
@@ -704,7 +704,10 @@
 			{
 				memcpy(&result.rendered_data[0], &rendered_data[0], rendered_data.size());
 			}
-			countIsolines(result);
+			if (result.rendered_data.size() > 0)
+			{
+				countIsolines(result);
+			}
 
 			/* Store the test run descriptor. */
 			m_test_results[test.vertex_spacing_mode].push_back(result);