Description of problem:
I hit a segfault when deleting a subvolume with very long name(>4096),
it's because cmd_subvol_delete() calls strdup() and passes NULL as
argument, which is returned by realpath(3).

I used the following script to reproduce

        #!/bin/bash
        mnt=$1
        i=1
        path=$mnt/subvol_$i

        # Create very deep subvolumes
        while btrfs sub create $path;do
              ((i++))
              path="$path/subvol_$i"
        done
        last_vol=$(dirname $path)
        dir=$(dirname $last_vol)
        vol=$(basename $last_vol)

        # Try to delete tha last one, this would get segfault
        pushd $dir
        btrfs sub delete $vol
        popd

Version-Release number of selected component (if applicable):
btrfs-progs-0.20.rc1.20130917git194aa4a-1.el7

How reproducible:
always

Steps to Reproduce:
1. see description
2.
3.

Actual results:
btrfs segfaults

Expected results:
subvolume can be deleted

Additional info: