mapistore/backend_get_path: Check return value from backend.get_path for != NULL

Backend may return MAPISTORE_SUCCESS and dbpath = NULL, in which
case are going to make a fake url like:
  sogo://(null)
Such broken URLs quickly polute both indexing and openchangedb databases.

Signed-off-by: Kamen Mazdrashki <kmazdrashki@zentyal.com>
Merge pull request #63 from Zentyal/kamenim/check-mapistore_backend_path

mapistore/backend_get_path: Check return value from backend.get_path for != NULL