Fix/Workaround T49297: Crash related to custom data draw (Blender with ASAN)

Root of the issue is that active render index became wrong. This is the actual
thing to be fixed, but as usual this is quite tricky to reproduce. Since such
bad situation might have happened more and fix isn't really difficult or
intruisive let's avoid crash for now.

Can be revisited once we figure out root of the issue.

Nice for 2.78 release.

1 files changed, 8 insertions, 4 deletions

diff --git a/source/blender/blenkernel/intern/DerivedMesh.c b/source/blender/blenkernel/intern/DerivedMesh.c
index 5f759c61bcb..8168817491f 100644
--- a/source/blender/blenkernel/intern/DerivedMesh.c
+++ b/source/blender/blenkernel/intern/DerivedMesh.c
@@ -3474,13 +3474,17 @@ void DM_calc_loop_tangents(
 
 		/* Update active layer index */
 		uv_index = CustomData_get_layer_index_n(&dm->loopData, CD_MLOOPUV, act_uv_n);
-		tan_index = CustomData_get_named_layer_index(&dm->loopData, CD_TANGENT, dm->loopData.layers[uv_index].name);
-		CustomData_set_layer_active_index(&dm->loopData, CD_TANGENT, tan_index);
+		if (uv_index != -1) {
+			tan_index = CustomData_get_named_layer_index(&dm->loopData, CD_TANGENT, dm->loopData.layers[uv_index].name);
+			CustomData_set_layer_active_index(&dm->loopData, CD_TANGENT, tan_index);
+		}
 
 		/* Update render layer index */
 		uv_index = CustomData_get_layer_index_n(&dm->loopData, CD_MLOOPUV, ren_uv_n);
-		tan_index = CustomData_get_named_layer_index(&dm->loopData, CD_TANGENT, dm->loopData.layers[uv_index].name);
-		CustomData_set_layer_render_index(&dm->loopData, CD_TANGENT, tan_index);
+		if (uv_index != -1) {
+			tan_index = CustomData_get_named_layer_index(&dm->loopData, CD_TANGENT, dm->loopData.layers[uv_index].name);
+			CustomData_set_layer_render_index(&dm->loopData, CD_TANGENT, tan_index);
+		}
 	}
 }