pgsql: postgres_fdw: Replace buffers in RemoteAttributeMapping with poi

postgres_fdw: Replace buffers in RemoteAttributeMapping with pointers.

Commit 28972b6fc ("Add support for importing statistics from remote
servers.") stored the names of local/remote columns for a foreign table
into the buffers of NAMEDATALEN bytes in this structure, without
accounting for the possibility that the remote column name in particular
could be longer than NAMEDATALEN - 1. If it was longer than that, this
would leave it unterminated/truncated in the buffer, invoking undefined
behavior when match_attrmap() processes it, which assumes that it's
fully-contained/terminated in the buffer.

To fix, replace the buffers with char pointers, pstrdup the local/remote
column names, and store the results into the pointers. This commit also
adds a function to clean up the nested data structure.

Per Coverity and Tom Lane.

Reported-by: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>
Author: Corey Huinker <corey(dot)huinker(at)gmail(dot)com>
Reviewed-by: Etsuro Fujita <etsuro(dot)fujita(at)gmail(dot)com>
Discussion: https://postgr.es/m/342868.1776017700%40sss.pgh.pa.us

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/aa1f93a3387ad619c14cea2b8ed01e6f49cb6600

Modified Files
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contrib/postgres_fdw/postgres_fdw.c | 32 ++++++++++++++++++++++++++------
1 file changed, 26 insertions(+), 6 deletions(-)