Thanks to Tim Waugh <twaugh redhat com> for the patch
Prevent crash when DEVICE_URI/PRINTER environment variables are not
  set (https://bugzilla.redhat.com/show_bug.cgi?id=479808 comment 6)

diff -Nur hplip-3.9.4b.old/prnt/hpijs/hpijs.cpp hplip-3.9.4b.new/prnt/hpijs/hpijs.cpp
--- hplip-3.9.4b.old/prnt/hpijs/hpijs.cpp	2009-04-29 15:36:54.000000000 -0500
+++ hplip-3.9.4b.new/prnt/hpijs/hpijs.cpp	2009-06-09 00:41:44.684172840 -0500
@@ -218,8 +218,14 @@
 		// call dbus here
 		const char    *user_name = " ";
 		const char    *title     = " ";
+		char *device_uri = getenv ("DEVICE_URI");
+		char *printer = getenv("PRINTER");
 		int     job_id = 0;
-                SendDbusMessage (getenv ("DEVICE_URI"), getenv("PRINTER"),
+		if (!device_uri)
+		    device_uri = "";
+		if (!printer)
+		    printer = "";
+                SendDbusMessage (device_uri, printer,
 	     	                 EVENT_PRINT_FAILED_MISSING_PLUGIN,
 				 user_name, job_id, title);
                 BUG("unable to set device=%s, err=%d\n", svalue, r);