logging: the 'log backend' shell cmd will result in the infinite loop.

The backend_name_get() function always sets 'entry->syntax' to
the name of the last backend, but a calling function iterates
over the idx until 'entry->syntax' is null.
Solution: Added a check that the index is less than the number
of backends and added getting the name of the backend
by the given index.

Signed-off-by: Mikhail Siomin <victorovich.01@mail.ru>