libglusterfs: avoid the logging which says the port is invalid

If the reserved ports file in proc contains just a newline, then
do not proceed with ports checking and reserving.

Change-Id: I776d0be1c3824dcd982f0685b171f2172b4e11e6
BUG: 762989
Signed-off-by: Raghavendra Bhat <raghavendra@redhat.com>
Reviewed-on: http://review.gluster.org/4821
Tested-by: Gluster Build System <jenkins@build.gluster.com>
Reviewed-by: Vijay Bellur <vbellur@redhat.com>