BUG #15041: dsa alloc_object null pointer

The following bug has been logged on the website:

Bug reference: 15041
Logged by: Daniel Farina
Email address: daniel(at)fdr(dot)io
PostgreSQL version: 10.1
Operating system: Linux
Description:

A database that was operating normally for quite a while suddenly generated
three similar looking core-dumps near one another. The stack traces look
like this.

It is possible there was unusual memory pressure, at the time this occurred.
This is the first occurrence.

#0 alloc_object (size_class=<optimized out>, area=0x0) at dsa.c:1433
#1 dsa_allocate_extended (area=0x0, size=size(at)entry=72,
flags=flags(at)entry=4) at dsa.c:785
#2 0x000000000062d277 in tbm_prepare_shared_iterate
(tbm=tbm(at)entry=0x1e54160) at tidbitmap.c:807
#3 0x00000000005f69a0 in BitmapHeapNext (node=node(at)entry=0x1d22a48) at
nodeBitmapHeapscan.c:155
#4 0x00000000005ebe63 in ExecScanFetch (recheckMtd=0x5f62c0
<BitmapHeapRecheck>, accessMtd=0x5f6320 <BitmapHeapNext>, node=0x1d22a48) at
execScan.c:97
#5 ExecScan (node=0x1d22a48, accessMtd=0x5f6320 <BitmapHeapNext>,
recheckMtd=0x5f62c0 <BitmapHeapRecheck>) at execScan.c:164
#6 0x000000000060402f in ExecProcNode (node=0x1d22a48) at
../../../src/include/executor/executor.h:250
#7 ExecNestLoop (pstate=0x1d22888) at nodeNestloop.c:109
#8 0x0000000000606506 in ExecProcNode (node=0x1d22888) at
../../../src/include/executor/executor.h:250
#9 ExecSort (pstate=0x1d22618) at nodeSort.c:106
#10 0x00000000005fed83 in ExecProcNode (node=0x1d22618) at
../../../src/include/executor/executor.h:250
#11 gather_merge_readnext (gm_state=0x1d223a8, reader=<optimized out>,
nowait=<optimized out>) at nodeGatherMerge.c:631
#12 0x00000000005ff06d in gather_merge_init (gm_state=0x1d223a8) at
nodeGatherMerge.c:468
#13 gather_merge_getnext (gm_state=0x1d223a8) at nodeGatherMerge.c:536
#14 ExecGatherMerge (pstate=0x1d223a8) at nodeGatherMerge.c:250
#15 0x00000000005fdfc9 in ExecProcNode (node=0x1d223a8) at
../../../src/include/executor/executor.h:250
#16 ExecLimit (pstate=0x1d21c38) at nodeLimit.c:95
#17 0x00000000005e6672 in ExecProcNode (node=0x1d21c38) at
../../../src/include/executor/executor.h:250
#18 ExecutePlan (execute_once=<optimized out>, dest=0x1ddc088,
direction=<optimized out>, numberTuples=0, sendTuples=<optimized out>,
operation=CMD_SELECT,
use_parallel_mode=<optimized out>, planstate=0x1d21c38,
estate=0x1d21988) at execMain.c:1722
#19 standard_ExecutorRun (queryDesc=0x161b1f8, direction=<optimized out>,
count=0, execute_once=<optimized out>) at execMain.c:363
#20 0x00007f7f42c12fd5 in pgss_ExecutorRun (queryDesc=0x161b1f8,
direction=ForwardScanDirection, count=0, execute_once=<optimized out>) at
pg_stat_statements.c:889
#21 0x000000000070850c in PortalRunSelect (portal=portal(at)entry=0x135a8e8,
forward=forward(at)entry=1 '\001', count=0, count(at)entry=9223372036854775807,
dest=dest(at)entry=0x1ddc088)
at pquery.c:932
#22 0x0000000000709880 in PortalRun (portal=portal(at)entry=0x135a8e8,
count=count(at)entry=9223372036854775807, isTopLevel=isTopLevel(at)entry=1 '\001',

run_once=run_once(at)entry=1 '\001', dest=dest(at)entry=0x1ddc088,
altdest=altdest(at)entry=0x1ddc088, completionTag=0x7fff5626f4d0 "") at
pquery.c:773
#23 0x00000000007059f9 in exec_simple_query (