nir: abort io info gathering if location is not set

Unlike spirv glsl varyings might not have explicit locations set. nir_shader_gather_info() was once only called at the end of linking but these days it even gets called in NIR optimisation loops via nir_opt_phi_precision.

In a following series I'm implementing a NIR version of the GLSL varying linker which means we will have varyings with no location set when nir_shader_gather_info() gets called the first few times.

Here rather than asserting we simply abort the io info gathering.