Fix zdb "Memory fault" found on FreeBSD ZTS (#16332)

Reason: nvlist_free() tries to free sth. which isn't allocted
Solution: init this variable with NULL

Closes #16311
Signed-off-by: Tino Reichardt <milky-zfs@mcmilk.de>
Reviewed-by: Alexander Motin <mav@FreeBSD.org>
Reviewed-by: Ameer Hamza <ahamza@ixsystems.com>
Reviewed-by: Tony Hutter <hutter2@llnl.gov>

cmd/zdb/zdb.c

@@ -9186,7 +9186,7 @@ main(int argc, char **argv)
 		char *pname = strdup(target);
 		const char *value;
 		nvlist_t *pnvl = NULL;
-		nvlist_t *vnvl;
+		nvlist_t *vnvl = NULL;
 
 		if (strpbrk(pname, "/@") != NULL)
 			*strpbrk(pname, "/@") = '\0';