Avoid double stringification in pp_complement

Commit 91bba34 made targets exempt from uninit warnings.  The com-
mit message only says it has something to do with fixing warnings on
bitwise ops.

Commit a1afd10 worked around that by doing an extra stringifica-
tion before assigning to the target, to produce an uninit warning
on purpose.

As far as I can tell, the only purpose of 91bba34 was to avoid the
warning for ~undef.  (I haven’t actually tried building the commit
before that to confirm.)

In any case, the uninit warning has been long tested for and is now
expected behaviour.

Since I am about to remove the uninit warning exemption for targets,
stop relying on that.

This speed ups the code slightly, as we avoid a double string-
ification.

pp.c

@@ -2315,9 +2315,8 @@ PP(pp_complement)
 	I32 anum;
 	STRLEN len;
 
-	(void)SvPV_nomg_const(sv,len); /* force check for uninit var */
-	sv_setsv_nomg(TARG, sv);
-	tmps = (U8*)SvPV_force_nomg(TARG, len);
+	sv_copypv_nomg(TARG, sv);
+	tmps = (U8*)SvPV_nomg(TARG, len);
 	anum = len;
 	if (SvUTF8(TARG)) {
 	  /* Calculate exact length, let's not estimate. */