errors when reading a log record which has a 0-length header
(like LOGREC_REDO_DROP_TABLE).


storage/maria/ma_loghandler.c:
  Functions reading record's header now don't use 0 to indicate error,
  as some valid records have a 0-length header (like REDO_DROP_TABLE).
  Instead, negative values are used for EOF and error.
storage/maria/ma_loghandler.h:
  functions to read record's header now return an int
  (either the length of this header (>=0) or some negative values
  for EOF or error).
storage/maria/ma_recovery.c:
  update to the new log handler's behaviour. Note the @todo.
storage/maria/maria_read_log.c:
  inform when program failed
storage/maria/unittest/ma_test_loghandler-t.c:
  update to new log handler's API
storage/maria/unittest/ma_test_loghandler_multigroup-t.c:
  update to new log handler's API
storage/maria/unittest/ma_test_loghandler_multithread-t.c:
  update to new log handler's API