Fix zdb "Memory fault" found on FreeBSD ZTS

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

Signed-off-by: Tino Reichardt <milky-zfs@mcmilk.de>
Closes #16311

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';