Tftp: tftp-hpa: denial of service due to out-of-bounds read/write in remap engine

A flaw was found in tftp-hpa. When the `in.tftpd` remap engine processes an inverse remap rule that also aborts with a non-empty custom error message, it can pass invalid match offsets to the `genmatchstring()` function. This leads to out-of-bounds read/write operations. A remote, unauthenticated attacker can exploit this vulnerability by sending a specially crafted request, causing the daemon to crash and resulting in a denial of service.