Fix bug in `expr_clone`

Fix bug in `expr_clone`

On allocation failure of nhls for EXPR_OP_CALL2,
EXPR_OP_UP, EXPR_OP_CALL1, the correct behaviour,
that is returning -1, was only effective if

  node->kind == EXPR_OP_CALL2 and node->u.call.own_rhs

This change implements the correct behaviour.

It also solves a warning when compiling with gcc4.9 that
broke integration when compiling with -Werror: the compiler
was hurt to use nrhs that he suspected (wrongly) to be
uninitialised.

Change-Id: I184fea5e121d38fa1df5d0a8680edf51e9fff1a8
Signed-off-by: José Bollo <jose.bollo@open.eurogiciel.org>