libdpkg: Reset dblockfd on modstatdb_unlock()

This will allow locking again the database after having previously
unlocked it.

lib/dpkg/dbmodify.c

@@ -229,6 +229,8 @@ modstatdb_unlock(void)
 {
   /* Unlock. */
   pop_cleanup(ehflag_normaltidy);
+
+  dblockfd = -1;
 }
 
 enum modstatdb_rw