DTP: improve packet integrity checks

Adjust the TLV infinite loop check to require the right amount of bytes
for T and L; do it after printing the TLV header so it is easier to
understand what was wrong. Check if the TLV V is within the capture. Use
the new "invalid" exit label to indicate a packet anomaly and add more
checks for the length value into the type-specific case blocks. Print
the domain string with fn_printzp().