s390-tools/utils: handle util_scandir errors correctly

The util_scandir_* functions may return 0 or -1, in which case no vector
is allocated in libutil/util_scandir.c. util_ptr_vec_free, called by
util_scandir_free or directly from lschp.c and lsscm.c, does always
call free for the vector which might be not initialized.

Fix this by always initializing the vector with NULL in __scandir and
add some api hardening by checking the vector and count in
util_ptr_vec_free before iterating over the vector.
And update the comment for util_scandir to indicate that -1 may be
returned in error cases plus that the vector is initialized with NULL.

Fixes: https://github.com/ibm-s390-tools/s390-tools/issues/43
Reported-by: Cornelia Huck <cohuck@redhat.com>
Reviewed-by: Jan Höppner <hoeppner@linux.ibm.com>
Signed-off-by: Karsten Graul <kgraul@linux.ibm.com>
Signed-off-by: Jan Höppner <hoeppner@linux.ibm.com>

include/lib/util_base.h

@@ -42,6 +42,9 @@ static inline void util_ptr_vec_free(void **ptr_vec, int count)
 {
 	int i;
 
+	if (!ptr_vec || count < 0)
+		return;
+
 	for (i = 0; i < count; i++)
 		free(ptr_vec[i]);
 	free(ptr_vec);