Using a MEMORY table BTREE index for scanning for updatable rows
could lead to an infinite loop.

Everytime a key was inserted into a btree index, the position
in the index scan was cleared. The search started from the
beginning and found the same key again.

Now we do not clear the position on key insert an more.


heap/hp_write.c:
  Bug#26996 - Update of a Field in a Memory Table ends with wrong result
  Removed the index-scan-breaking nulling of last_pos.
  The comment behind this line ("For heap_rnext/heap_rprev")
  was misleading. It should have been "Breaks heap_rnext/heap_rprev".
mysql-test/r/heap_btree.result:
  Bug#26996 - Update of a Field in a Memory Table ends with wrong result
  Added test result.
mysql-test/t/heap_btree.test:
  Bug#26996 - Update of a Field in a Memory Table ends with wrong result
  Added test.