intel/compiler: Don't fallback to vec4 when scalar GS compile fails [v2]

v2: Add missing error string handling.  Noticed by Jason.

Reviewed-by: Jason Ekstrand <jason@jlekstrand.net>
Reviewed-by: Matt Turner <mattst88@gmail.com> [v1]
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/6826>

src/intel/compiler/brw_vec4_gs_visitor.cpp

@@ -872,6 +872,11 @@ brw_compile_gs(const struct brw_compiler *compiler, void *log_data,
          g.add_const_data(nir->constant_data, nir->constant_data_size);
          return g.get_assembly();
       }
+
+      if (error_str)
+         *error_str = ralloc_strdup(mem_ctx, v.fail_msg);
+
+      return NULL;
    }
 
    if (compiler->devinfo->gen >= 7) {