The test started failing following the push for BUG#41541.
Some of the algorithms access bytes beyond the input data
and this can affect up to one byte less than "word size"
which is BITS_SAVED / 8. 
      
Fixed by adding (BITS_SAVED / 8) -1 bytes to buffer size
(i.e. Memory Segment #2) to avoid accessing un-allocated data.

myisam/mi_packrec.c:
  Fixed _mi_read_pack_info() method to allocate (BITS_SAVED/8) - 1 
  bytes to the Memory Segment #2
mysql-test/r/myisampack.result:
  Result file for BUG#43973
mysql-test/t/myisampack.test:
  Testcase for BUG#43973