single quote fails in 5.1.x

Performing fulltext prefix search (a word with truncation
operator) may cause a dead-loop.

The problem was in smarter index merge algorithm - it was writing
record reference to an incorrect memory area.

mysql-test/r/fulltext.result:
  A test case for BUG#42907.
mysql-test/t/fulltext.test:
  A test case for BUG#42907.
storage/myisam/ft_boolean_search.c:
  Fixed incorrect memory update by _mi_dpointer() when performing
  fulltext prefix search.