DHCP broken in libslirp v4.6.0

DHCP got broken in v4.6.0.

Easily reproducible with QEMU, but can be reproducible with slirp4netns too.

$ rootlesskit --net=slirp4netns udhcpc -f -i tap0
WARN[0000] specifying --disable-host-loopback is highly recommended to prohibit connecting to 127.0.0.1:* on the host namespace (requires slirp4netns or VPNKit) 
WARN[0000] Mounting /etc/resolv.conf without copying-up /etc. Note that /etc/resolv.conf in the namespace will be unmounted when it is recreated on the host. Unless /etc/resolv.conf is statically configured, copying-up /etc is highly recommended. Please refer to RootlessKit documentation for further information. 
udhcpc: started, v1.30.1
udhcpc: sending discover
udhcpc: packet with bad magic, ignoring

libslirp 4.5.0 was working as expected.

$ rootlesskit --net=slirp4netns udhcpc -f -i tap0
WARN[0000] specifying --disable-host-loopback is highly recommended to prohibit connecting to 127.0.0.1:* on the host namespace (requires slirp4netns or VPNKit) 
WARN[0000] Mounting /etc/resolv.conf without copying-up /etc. Note that /etc/resolv.conf in the namespace will be unmounted when it is recreated on the host. Unless /etc/resolv.conf is statically configured, copying-up /etc is highly recommended. Please refer to RootlessKit documentation for further information. 
udhcpc: started, v1.30.1
udhcpc: sending discover
udhcpc: sending select for 10.0.2.15
udhcpc: lease of 10.0.2.15 obtained, lease time 86400

Regression in commit f13cad45