glsl: always return true at the end of link_varyings

Require that the function returns false before we get to the end if there
is any linker error.

Reviewed-by: Alejandro Piñeiro <apinheiro@igalia.com>
Reviewed-by: Pierre-Eric Pelloux-Prayer <pierre-eric.pelloux-prayer@amd.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/34307>

src/compiler/glsl/gl_nir_link_varyings.c

@@ -4555,7 +4555,8 @@ link_varyings(struct gl_shader_program *prog, unsigned first,
                              has_xfb_qualifiers, mem_ctx))
       return false;
 
-   return prog->data->LinkStatus != LINKING_FAILURE;
+   assert(prog->data->LinkStatus != LINKING_FAILURE);
+   return true;
 }
 
 bool