SIGABRT - hpcups crashes at destructor

Hi!

Marek Kasik reported a bug https://bugzilla.redhat.com/show_bug.cgi?id=1695716 for hplip in Fedora together with patch. The crash happens when user installs HP LaserJet P1006, but without proprietary plugin and tries to print something.

It is reproducible even with file device printer. Setup:

1) enable FileDevice in /etc/cups/cups-files.conf
2) restart cups
3) create a print queue:
$ sudo lpadmin -p test -v file:/tmp/ps -m drv:///hp/hpcups.drv/hp-laserjet_p1006.ppd -E
4) print something, e.g. /etc/fstab
$ lp -d test /etc/fstab

Now you can see in logs/journal:
Apr 04 09:08:26 host-10-0-136-26 cupsd[17313]: prnt/hpcups/HPCupsFilter.cpp 490: m_Job initialization failed with error = 48
Apr 04 09:08:26 host-10-0-136-26 cupsd[17313]: PID 17319 (/usr/lib/cups/filter/hpcups) crashed on signal 11.
Apr 04 09:08:26 host-10-0-136-26 cupsd[17313]: Hint: Try setting the LogLevel to "debug" to find out more.
Apr 04 09:08:26 host-10-0-136-26 cupsd[17313]: Processing page 2...
Apr 04 09:08:26 host-10-0-136-26 cupsd[17313]: Rendering completed
Apr 04 09:08:26 host-10-0-136-26 cupsd[17313]: PID 17318 (/usr/lib/cups/filter/gstoraster) exited with no errors.

The issue is similar like https://bugs.launchpad.net/bugs/839632 , but for other variable - compressBuf - the fix is to initiliaze it to NULL.

Would you mind adding the patch to the project?