core: fix compilation with gcc -O3

core: fix compilation with gcc -O3

../src/core/path.c: In function ‘path_serialize’:
../src/core/path.c:616:24: error: ‘%s’ directive argument is null [-Werror=format-overflow=]
  616 |                 (void) serialize_item_format(f, "path-spec", "%s %%i %%s",
      |                        ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
  617 |                                              path_type_to_string(s->type) //,
      |                                              ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
  618 |                                              //                                             s->previous_exists,
      |                                              ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
  619 |                                              //                                             s->path
      |                                              ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
  620 |                 );
      |                 ~
In function ‘path_spec_dump’,
    inlined from ‘path_dump’ at ../src/core/path.c:392:17:
../src/core/path.c:226:9: error: ‘%s’ directive argument is null [-Werror=format-overflow=]
  226 |         fprintf(f,
      |         ^~~~~~~~~~
  227 |                 "%s%s: %s\n",
      |                 ~~~~~~~~~~~~~
  228 |                 prefix,
      |                 ~~~~~~~
  229 |                 path_type_to_string(s->type),
      |                 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
  230 |                 s->path);
      |                 ~~~~~~~~

s->type should be valid here, so let's just add an assert.

For https://github.com/systemd/systemd/issues/6119#issuecomment-626073743.