dev->ip_ptr could be NULL if we set an invalid MTU.

Even then, if we issue ioctl(SIOCSIFADDR) for a new IPv4 address,
devinet_ioctl() allocates struct in_ifaddr and fails later in
inet_set_ifa() because in_dev is NULL.

Let's move the check earlier.
Signed-off-by: Kuniyuki Iwashima <kuniyu@amazon.com>
Link: https://patch.msgid.link/20240809235406.50187-2-kuniyu@amazon.comSigned-off-by: Jakub Kicinski <kuba@kernel.org>