Fix Use-After-Free issue for Regexp

Co-authored-by: Isaac Peka <7493006+isaac-peka@users.noreply.github.com>

regexec.c

@@ -3435,8 +3435,8 @@ match_at(regex_t* reg, const UChar* str, const UChar* end,
     CASE(OP_MEMORY_END_PUSH_REC)  MOP_IN(OP_MEMORY_END_PUSH_REC);
       GET_MEMNUM_INC(mem, p);
       STACK_GET_MEM_START(mem, stkp); /* should be before push mem-end. */
-      STACK_PUSH_MEM_END(mem, s);
       mem_start_stk[mem] = GET_STACK_INDEX(stkp);
+      STACK_PUSH_MEM_END(mem, s);
       MOP_OUT;
       JUMP;