btrfs-progs: fix test for return of realpath in find_mount_root()

find_mount_root() tries to test for realpath() failure, but
tests the wrong value.  Fix it.

Resolves-Coverity-CID: 1125940
Signed-off-by: Eric Sandeen <sandeen@redhat.com>
Reviewed-by: Anand Jain <anand.jain@oracle.com>
Signed-off-by: Chris Mason <chris.mason@fusionio.com>

cmds-send.c

@@ -98,7 +98,7 @@ int find_mount_root(const char *path, char **mount_root)
 
 	ret = 0;
 	*mount_root = realpath(longest_match, NULL);
-	if (!mount_root)
+	if (!*mount_root)
 		ret = -errno;
 
 	free(longest_match);