fix 'vfprintf %s NULL'

Index: src/archive_manager.cc
--- src/archive_manager.cc.orig
+++ src/archive_manager.cc
@@ -95,7 +95,8 @@ describe(const fs::path& filename)
 
         const auto* format_name = archive_format_name(arc);
 
-        log_debug("read next header %s %s", format_name, filename.c_str());
+        if (format_name != nullptr)
+            log_debug("read next header %s %s", format_name, filename.c_str());
         if (archive_read_next_header(arc, &entry) == ARCHIVE_OK) {
             log_debug("read next done %s", filename.c_str());
 
