Address CVE-2018-17141 and fixes a few vulnerabilities in code supporting JPEG master

Address CVE-2018-17141 and fixes a few vulnerabilities in code supporting JPEG

These changes are adapted from Lee's fix for this vulnerability.

Luis Merino, Markus Vervier, and Eric Sesterhenn of X41 D-SEC GmbH
(Security Advisory: X41-2018-008) discovered an uninitialized pointer write
and also an out-of-bounds write in FaxModem::writeECMData() that could lead
to remote code execution with a specially-crafted fax sender.

These changes fix the coding errors and deliberately prevent malicious and
malfunctioning senders from inadvertently or deliberately setting JPEG and
MH/MR/MMR/JBIG formats in the same DCS signal.