MDEV-21749: page_cur_insert_rec_low(): Assertion rdm - rd + bd <= insert_buf + rec_size failed.

This bug was introduced in
commit 7ae21b18
(the main commit of MDEV-12353).

page_cur_insert_rec_low(): Before entering the comparison loop, make sure
that the range does not exceed c_end already at the start of the loop.
The loop is only comparing for pointer equality, and that condition
cdm == c_end would never hold if the end was already exceeded in
the beginning. Also, skip the comparison altogether if we could find
at most 2 equal bytes.

PageBulk::insertPage(): Apply a similar change. It seems that this
code was correct, because the loop checks for cdm < c_end.

storage/innobase/btr/btr0bulk.cc

@@ -275,6 +275,8 @@ inline void PageBulk::insertPage(rec_t *rec, offset_t *offsets)
           goto no_data;
 
       /* Try to copy any data bytes of the preceding record. */
+      if (c_end - cd > 2)
+      {
         const byte *cdm= cd;
         const byte *rdm= rd;
         for (; cdm < c_end && *rdm == *cdm; cdm++, rdm++)
@@ -291,6 +293,7 @@ inline void PageBulk::insertPage(rec_t *rec, offset_t *offsets)
           r= rdm;
         }
       }
+    }
 
     if (size_t len= static_cast<size_t>(insert_rec_end - b))
       m_mtr.memcpy<mtr_t::FORCED>(*m_block, b, r, len);

storage/innobase/page/page0cur.cc

@@ -1342,6 +1342,8 @@ page_cur_insert_rec_low(
 
         /* Try to copy any data bytes of the preceding record. */
         const byte * const cd= cur->rec + (rd - rec);
+        if (c_end - cd > 2)
+        {
           const byte *cdm= cd;
           const byte *rdm= rd;
           while (*rdm++ == *cdm++)
@@ -1362,6 +1364,7 @@ page_cur_insert_rec_low(
           }
         }
       }
+    }
 
     if (size_t len= static_cast<size_t>(insert_buf + rec_size - b))
       mtr->memcpy<mtr_t::FORCED>(*block, b, r, len);