foreign table: WARNING: problem in alloc set ExecutorState detected write past chunk end in block ...

While trying to test out the "EXPLAIN and nfiltered, take two" patch
with its test file "explaintesti", I noticed I was getting a warning
message like:

WARNING: problem in alloc set ExecutorState: detected write past
chunk end in block 0x101134e00, chunk 0x101134f38

I got the same warning on unpatched git head as well. I was able to
reproduce the problem with the attached input.sql file, along with a
dummy input file, dummy_passwd, for the foreign table to read from. I
get the above warning along with:

ERROR: missing data for column "pass"
CONTEXT: COPY passwd, line 1: "##"
STATEMENT: SELECT * FROM passwd WHERE uid > 4;

The error message is not surprising, since I am feeding the foreign
table improperly-formatted lines. The warning is what concerns me;
from the comments in aset.c and the warning message text itself, I
gather this is not expected. I tested on OS X 10.6, with
--enable-cassert --enable-debug.

Josh