cifs: Fix an error pointer dereference in cifs_mount()

The error handling calls kfree(full_path) so we can't let it be a NULL
pointer.  There used to be a NULL assignment here but we accidentally
deleted it.  Add it back.

Fixes: 7efd0815 ("cifs: document and cleanup dfs mount")
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Steve French <stfrench@microsoft.com>
Reviewed-by: Paulo Alcantara (SUSE) <pc@cjr.nz>

fs/cifs/connect.c

@@ -4886,6 +4886,7 @@ int cifs_mount(struct cifs_sb_info *cifs_sb, struct smb_vol *vol)
 		full_path = build_unc_path_to_root(vol, cifs_sb, !!count);
 		if (IS_ERR(full_path)) {
 			rc = PTR_ERR(full_path);
+			full_path = NULL;
 			break;
 		}
 		/* Chase referral */