A bug in the initialization of key segment information made it point
to the wrong bit, since a bit index was used when its int value
was needed. This lead to misinterpretation of bit columns
read from MyISAM record format when a NULL bit pushed them over
a byte boundary.
Fixed by using the int value of the bit instead.


mysql-test/r/myisam.result:
  Bug#43737: Test result.
mysql-test/t/myisam.test:
  Bug#43737: Test case.
storage/myisam/mi_open.c:
  Bug#43737: fix.