Blender Git Loki

Avoid Assert in BKE_mesh_calc_loop_tangent_ex

Code could call CustomData_get_layer_index_n with a negative index (if
no active and/or render UV layers are found). This would assert since
rBe86785c51445.

Spotted while looking into T81398.

Differential Revision: https://developer.blender.org/D9212