gfapi: ignore NULL and -1 parameters in glfs_set_logging()

Permit changing either logfile or level by specifying the other
as -1 or NULL respectively.

Change-Id: I7df05db7b613c57e0106a5cce846ae26d2a160d4
BUG: 953694
Signed-off-by: Anand Avati <avati@redhat.com>
Reviewed-on: http://review.gluster.org/5085
Reviewed-by: Vijay Bellur <vbellur@redhat.com>
Tested-by: Gluster Build System <jenkins@build.gluster.com>