test: fix printf("%s", NULL) in error path

test: fix printf("%s", NULL) in error path

    ../test/common.c: In function ‘test_get_path’:
    ../test/common.c:171:9: warning: ‘%s’ directive argument is null [-Wformat-overflow=]
      171 |         fprintf(stderr, "Failed to allocate path (%d chars) for %s\n",
          |         ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
      172 |                 (int) path_len, path);
          |                 ~~~~~~~~~~~~~~~~~~~~~

Signed-off-by: Ran Benita <ran@unusedvar.com>