libglusterfs: set inode path to be NULL in case of errors

Some of the functions calling inode_path, __inode_path
are assuming the path to be set to NULL in case of errors.
Instead of fixing it in the calling functions, fixing it in
the __inode_path function.

Change-Id: I77736a2700d3c2c9732a536bcf2a398fe626d54e
BUG: 765430
Signed-off-by: Pranith Kumar K <pranithk@gluster.com>
Reviewed-on: http://review.gluster.com/810
Tested-by: Gluster Build System <jenkins@build.gluster.com>
Reviewed-by: Jeff Darcy <jdarcy@redhat.com>
Reviewed-by: Vijay Bellur <vijay@gluster.com>

libglusterfs/src/inode.c

@@ -1079,6 +1079,8 @@ out:
                 }
         }
 
+        if (ret < 0)
+                *bufp = NULL;
         return ret;
 }