pgsql: Add static assertion to track the no-padding rule of varatt_exte

Add static assertion to track the no-padding rule of varatt_external_oid

varatt_external_oid (varatt_external in v19 and older branches) having
no padding is a rule that has been implied for years, as pointers of
this structure may be compared using memcpy().

I doubt that anybody is going to break this rule as varatt_external is
old enough that it could vote in most countries, but this makes varatt.h
more consistent for a follow-up change that introduces a second on-disk
external TOAST pointer, and it's free to have a compile-time rule.

Suggested-by: Bharath Rupireddy <bharath(dot)rupireddyforpostgres(at)gmail(dot)com>
Discussion: https://postgr.es/m/CALj2ACVG9cjxr-0jb8PP_Vb7vJvVGBVM_fQ2srt4CpyURpfYhQ@mail.gmail.com

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/1497ea77edd2ce261670f117301dc4da33d4c894

Modified Files
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src/include/varatt.h | 4 ++++
1 file changed, 4 insertions(+)