tree.c: avoid segfault with closing parenthesis ')' after -files0-from

Prompted by the following warning of GCC-11.1.1:

  tree.c: In function 'get_expr':
  tree.c:140:31: warning: dereference of NULL 'prev_pred' [CWE-476] \
  [-Wanalyzer-null-dereference]
    140 |       if ((UNI_OP == prev_pred->p_type
        |                      ~~~~~~~~~^~~~~~~~

Former versions of find are not vulnerable to this, because a closing
parenthesis ')' without anything before was treated as a pathname
rather than an option.
But this is possible now with the recent introduction of the -files0-from
option (see commit 11576f4e6a) - yet still an invalid invocation.
Reproducer for a crash:
  $ find -files0-from FILE ')' -print
  Segmentation fault (core dumped)

In the similar case when the user didn't specify any action, and find(1)
adds the default action via '( user-expr ) -print', the error diagnostic
was very confusing, too:
  $ find -files0-from FILE ')'
  find: invalid expression; empty parentheses are not allowed.

* find/tree.c (get_expr): Handle the cases when there's no predicate
before CLOSE_PAREN, and output a useful error diagnostic.
* tests/find/files0-from.sh: Add a test case for it.

0 files changed, 0 insertions, 0 deletions