gfapi: fix read past EOF in glfs_preadv()

When syncop_preadv() returns 0, we are returning @size as-is
(without modifying user passed buffer). This results in
junk data being read when read offset is at or beyond EOF.

Change-Id: I8aa9c957359f9a52833b944b452f501b6f5538d3
BUG: 953694
Signed-off-by: Anand Avati <avati@redhat.com>
Reviewed-on: http://review.gluster.org/5236
Reviewed-by: Raghavendra Talur <rtalur@redhat.com>
Reviewed-by: José Rivera <jrivera@redhat.com>
Tested-by: José Rivera <jrivera@redhat.com>
Tested-by: Gluster Build System <jenkins@build.gluster.com>
Reviewed-by: Vijay Bellur <vbellur@redhat.com>