The bug is actually a duplicate of the bug 14708.
Down-ported the fix for 14708 from 5.0.  
Merged the test case for bug 14708 from 5.0.


mysql-test/r/fulltext_left_join.result:
  Added a test case for bug #25637 (duplicate .of bug 14708).
  Merged the test case for bug 14708 from 5.0.
mysql-test/t/fulltext_left_join.test:
  Added a test case for bug #25637 (duplicate of bug 14708).
  Merged the test case for bug 14708 from 5.0.
sql/item_func.cc:
  Fixed bug #25637: LEFT JOIN with BOOLEAN FULLTEXT loses left table matches.
  The bug is actually a duplicate of the bug 14708.
  Down-ported the fix for 14708 from 5.0.