WIP fix Dumper.xs for perls before 5.17.0

Author: leonerd

dist/Data-Dumper/Dumper.xs

@@ -1392,9 +1392,9 @@ DD_dump(pTHX_ SV *val, const char *name, STRLEN namelen, SV *retval, HV *seenhv,
 # if !defined(PL_vtbl_vstring) && PERL_VERSION_LT(5,17,0)
 	    SV * const vecsv = sv_newmortal();
 #  if PERL_VERSION_LT(5,10,0)
-	    scan_vstring(mg->mg_ptr, vecsv);
+	    scan_vstring(vstr_pv, vecsv);
 #  else
-	    scan_vstring(mg->mg_ptr, mg->mg_ptr + mg->mg_len, vecsv);
+	    scan_vstring(vstr_pv, vstr_pv + vstr_len, vecsv);
 #  endif
 	    if (!sv_eq(vecsv, val)) goto integer_came_from_string;
 # endif