Overflow fixes to TN3270 dissector
Revert "Retry read() if it returns EAGAIN or EINTR"

This reverts commit 6692225fa93105a3ebb2c777d38a2c7281347191.
Check packet length to make sure that at least 5 bytes are there.
Merge pull request #137 from eaescob/TN3270_fixes

Tn3270 fixes