hw/net/vmxnet3: Fix guest-triggerable assert()

The assert() that checks for valid MTU sizes can be triggered by
the guest (e.g. with the reproducer code from the bug ticket
https://gitlab.com/qemu-project/qemu/-/issues/517 ). Let's avoid
this problem by simply logging the error and refusing to activate
the device instead.

Fixes: d05dcd9 ("net: vmxnet3: validate configuration values during activate")
Signed-off-by: Thomas Huth <thuth@redhat.com>
Cc: qemu-stable@nongnu.org
Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>
[Mjt: change format specifier from %d to %u for uint32_t argument]
(cherry picked from commit 90a0778)
Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>

hw/net/vmxnet3.c

@@ -1441,7 +1441,10 @@ static void vmxnet3_activate_device(VMXNET3State *s)
     vmxnet3_setup_rx_filtering(s);
     /* Cache fields from shared memory */
     s->mtu = VMXNET3_READ_DRV_SHARED32(d, s->drv_shmem, devRead.misc.mtu);
-    assert(VMXNET3_MIN_MTU <= s->mtu && s->mtu <= VMXNET3_MAX_MTU);
+    if (s->mtu < VMXNET3_MIN_MTU || s->mtu > VMXNET3_MAX_MTU) {
+        qemu_log_mask(LOG_GUEST_ERROR, "vmxnet3: Bad MTU size: %u\n", s->mtu);
+        return;
+    }
     VMW_CFPRN("MTU is %u", s->mtu);
 
     s->max_rx_frags =