Fix compilation with gcc 11 and glibc 2.33.

It is possible to have a switch statement value that causes an
invalid free. So set nrhs to a value that can be distinguished if
we are in the fall-through case.

expr.c: In function ‘expr_clone’:
expr.c:240:33: error: ‘nrhs’ may be used uninitialized in this function [-Werror=maybe-uninitialized]
  240 |                                 free(nrhs);
      |                                 ^~~~~~~~~~
cc1: all warnings being treated as errors

This is downstream Fedora patch: ltrace-0.7.91-rh1799619.patch
The downstream patch can be removed.

Signed-off-by: Carlos O'Donell <carlos@redhat.com>