ASA-2019-00306 – curl: TFTP receive buffer overflow

libcurl contains a heap buffer overflow in the function (`tftp_receive_packet()`) that recevives data from a TFTP server. It calls `recvfrom()` with the default size for the buffer rather than with the size that was used to allocate it. Thus, the content that might overwrite the heap memory is entirely controlled by the server. The flaw exists if the user selects to use a "blksize" of 504 or smaller (default is 512). The smaller size that is used, the larger the possible overflow becomes. Users chosing a smaller size than default should be rare as the primary use case for changing the size is to make it larger.