hw/net/net_tx_pkt: Fix crash detected by fuzzer

hw/net/net_tx_pkt: Fix crash detected by fuzzer

QEMU currently crashes when it's started like this:

cat << EOF | ./qemu-system-i386 -device vmxnet3 -nodefaults -qtest stdio
outl 0xcf8 0x80001014
outl 0xcfc 0xe0001000
outl 0xcf8 0x80001018
outl 0xcf8 0x80001004
outw 0xcfc 0x7
outl 0xcf8 0x80001083
write 0x0 0x1 0xe1
write 0x1 0x1 0xfe
write 0x2 0x1 0xbe
write 0x3 0x1 0xba
writeq 0xe0001020 0xefefff5ecafe0000
writeq 0xe0001020 0xffff5e5ccafe0002
EOF

It hits this assertion:

qemu-system-i386: ../qemu/hw/net/net_tx_pkt.c:453: net_tx_pkt_reset:
 Assertion `pkt->raw' failed.

This happens because net_tx_pkt_init() is called with max_frags == 0 and
thus the allocation

    p->raw = g_new(struct iovec, max_frags);

results in a NULL pointer that causes the

    assert(pkt->raw);

in net_tx_pkt_reset() to fail later. To fix this issue we can check
that max_raw_frags was not zero before asserting that pkt->raw is
a non-NULL pointer.

Buglink: https://bugs.launchpad.net/qemu/+bug/1890157
Message-Id: <20210715193219.1132571-1-thuth@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Reviewed-by: Pankaj Gupta <pankaj.gupta@ionos.com>
Signed-off-by: Thomas Huth <thuth@redhat.com>