Moby HyperKit uninitialized memory use in virtio-sock pci_vtsock_proc_tx

HyperKit is a toolkit for embedding hypervisor capabilities in an application. In versions 0.20210107, function pci_vtsock_proc_tx in virtio-sock can lead to to uninitialized memory use. In this situation, there is a check for the return value to be less or equal to VTSOCK_MAXSEGS, but that check is not sufficient because the function can return -1 if it finds an error it cannot recover from. Moreover, the negative return value will be used by iovec_pull in a while condition that can further lead to more corruption because the function is not designed to handle a negative iov_len. This issue may lead to a guest crashing the host causing a denial of service and, under certain circumstance, memory corruption. This issue is fixed in commit af5eba2360a7351c08dfd9767d9be863a50ebaba.