C: fix 5.8.x XPVBM strlen (test 11)

broken with 911c59c87e1f0 (wrong strlen for old-style bm strings)

-xpvbm_list[0].xpv_pv = savepvn("Can\000...", 260);
+xpvbm_list[0].xpv_pv = savepvn("Can\000...", 3);