Performing fulltext prefix search (a word with truncation
operator) may cause a dead-loop. ft_min_word_len value
doesn't matter actually.

The problem was introduced along with "smarter index merge"
optimization.

mysql-test/r/fulltext.result:
  A test case for BUG#50351.
mysql-test/t/fulltext.test:
  A test case for BUG#50351.
storage/myisam/ft_boolean_search.c:
  When going up to first-level tree, we need to restore docid[0],
  so it informs fulltext index merge not to enter this second-level
  tree again (avoiding dead-loop).