MDEV-29471 Buffer overflow in page_cur_insert_rec_low()

In commit 244fdc43 (MDEV-29438)
we made sure that if the preceding record is the page infimum record,
no more than 8 bytes will be read from it. But, if the data payload of
the being-inserted record is less than 8 bytes (this can happen in
secondary indexes), we must not compare all 8 bytes.

This was caught by a failure of the test gcol.innodb_virtual_basic
under MemorySanitizer and some builds with AddressSanitizer.

storage/innobase/page/page0cur.cc

@@ -1573,7 +1573,9 @@ page_cur_insert_rec_low(
   {
     const byte *r= rec;
     const byte *c= cur->rec;
-    const byte *c_end= c + (page_rec_is_infimum(c) ? 8 : data_size);
+    const byte *c_end= c + data_size;
+    if (page_rec_is_infimum(c) && data_size > 8)
+      c_end= c + 8;
     static_assert(REC_N_OLD_EXTRA_BYTES == REC_N_NEW_EXTRA_BYTES + 1, "");
     if (c <= insert_buf && c_end > insert_buf)
       c_end= insert_buf;