Test case for crashing jam

[mmadia]

In the following attached patch, %s will cause jam to crash

+		AddExpanerRule "application/x-7z-compressed" : .7z : "7za l %s" 
+			: "7za x -y %s" ;

Debug output:

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[tcsetpgrp failed in terminal_inferior: Invalid Argument]
Thread 51494 caused an exception: Segment violation
Reading symbols from /boot/system/runtime_loader...done.
Loaded symbols for /boot/system/runtime_loader
Reading symbols from /boot/system/lib/libroot.so...done.
Loaded symbols for /boot/system/lib/libroot.so
[tcsetpgrp failed in terminal_inferior: Invalid Argument]
[Switching to team /boot/common/bin/jam -q @nightly-raw (51494) thread jam (51494)]
0x002889e6 in vfprintf () from /boot/system/lib/libroot.so
(gdb) bt
#0  0x002889e6 in vfprintf () from /boot/system/lib/libroot.so
#1  0x00282063 in fprintf () from /boot/system/lib/libroot.so
#2  0x00207bf4 in write_jcache ()
#3  0x00207dbf in jcache_done ()
#4  0x0020595f in main ()
(gdb) 

[bonefish]

Fixed in hrev35719.