random crashes on -HEAD for a few days now

at least one of my buildfarm members (emu) is crashing on what seems
totally unrelated regression tests for a few days now:

http://buildfarm.postgresql.org/cgi-bin/show_history.pl?nm=emu&br=HEAD

it took me about 10 tries to reproduce that manually and I'm getting the
following stacktrace:

#0 varbit_out (fcinfo=0x88c75000) at varbit.c:549
549 x = *sp;
(gdb) bt
#0 varbit_out (fcinfo=0x88c75000) at varbit.c:549
#1 0x1c217930 in FunctionCall1 (flinfo=0x1, arg1=2294763520) at fmgr.c:1195
#2 0x1c036fae in printtup (slot=0x88c730b0, self=0x7ecf4bc8) at
printtup.c:326
#3 0x1c10ab13 in ExecSelect (slot=0x88c730b0, dest=0x88c75000,
estate=0x88c7301c) at execMain.c:1427
#4 0x1c10a8b3 in ExecutePlan (estate=0x88c7301c, planstate=0x88c731b4,
operation=CMD_SELECT, numberTuples=0, direction=-2000203776,
dest=0x7ecf4bc8) at execMain.c:1353
#5 0x1c109793 in ExecutorRun (queryDesc=0x7fc60574,
direction=ForwardScanDirection, count=0) at execMain.c:243
#6 0x1c19b917 in PortalRunSelect (portal=0x7fd9f01c, forward=1 '\001',
count=0, dest=0x7ecf4bc8) at pquery.c:943
#7 0x1c19b63e in PortalRun (portal=0x7fd9f01c, count=2147483647,
isTopLevel=1 '\001', dest=0x7ecf4bc8, altdest=0x7ecf4bc8,
completionTag=0xcfbd1e50 "") at pquery.c:797
#8 0x1c19709b in exec_simple_query (
query_string=0x7e4e301c "SELECT v,\n SUBSTRING(v FROM 2 FOR
4) AS sub_2_4,\n SUBSTRING(v FROM 7 FOR 13) AS sub_7_13,\n
SUBSTRING(v FROM 6) AS sub_6\n FROM VARBIT_TABLE;") at postgres.c:962
#9 0x1c199fe2 in PostgresMain (argc=4, argv=0x894395c0,
username=0x89439454 "mastermind") at postgres.c:3529
#10 0x1c171fbe in BackendRun (port=0x88ac1400) at postmaster.c:3177
#11 0x1c171864 in BackendStartup (port=0x88ac1400) at postmaster.c:2800
#12 0x1c16f8af in ServerLoop () at postmaster.c:1272
#13 0x1c16ee85 in PostmasterMain (argc=6, argv=0xcfbd224c) at
postmaster.c:1027
#14 0x1c12bf74 in main (argc=6, argv=0xcfbd224c) at main.c:188

Stefan