xfstests: Fix hang when mkfs.btrfs isn't present

My earlier patch (xfstests: handle new mkfs.btrfs -f option cleanly)
had a flaw in that if set_prog_path mkfs.btrfs returns nothing,
the grep will hang.

Test for that case to avoid it, and just return the empty string
in that case.

Reported-by: Rich Johnston <rjohnston@sgi.com>
Signed-off-by: Eric Sandeen <sandeen@redhat.com>
Reviewed-by: Rich Johnston <rjohnston@sgi.com>
Signed-off-by: Rich Johnston <rjohnston@sgi.com>

common.config

@@ -108,7 +108,7 @@ set_prog_path()
 set_btrfs_mkfs_prog_path_with_opts()
 {
 	p=`set_prog_path mkfs.btrfs`
-	if grep -q 'force overwrite' $p; then
+	if [ "$p" != "" ] && grep -q 'force overwrite' $p; then
 		echo "$p -f"
 	else
 		echo $p