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

[Eric Sandeen <sandeen@xxxxxxxxxxx>]

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@xxxxxxx>
Signed-off-by: Eric Sandeen <sandeen@xxxxxxxxxx>
---

diff --git a/common.config b/common.config
index c10163a..a0b017d 100644
--- a/common.config
+++ b/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