In Perl_reg_temp_copy(), ensure SvMAGIC() is NULL in the new REGEXP.

Change c2123ae exposed the fact that Perl_reg_temp_copy() didn't
reset SvMAGIC() to NULL after block copying the "parent" regexp. The analagous
problem with SvSTASH() was fixed with change b9ad13a.

Author: nwc10

regcomp.c

@@ -9461,6 +9461,7 @@ Perl_reg_temp_copy (pTHX_ REGEXP *ret_x, REGEXP *rx)
 	   sizeof(regexp) - STRUCT_OFFSET(regexp, xpv_cur));
     SvLEN_set(ret_x, 0);
     SvSTASH_set(ret_x, NULL);
+    SvMAGIC_set(ret_x, NULL);
     Newx(ret->offs, npar, regexp_paren_pair);
     Copy(r->offs, ret->offs, npar, regexp_paren_pair);
     if (r->substrs) {