pgsql: Remove wchar.c Asserts that were stricter than the main code

Remove wchar.c Asserts that were stricter than the main code

Assert errors were thrown for functions being passed invalid encodings,
while the main code handled it just fine.

Also document that libpq's PQclientEncoding() returns -1 for an encoding
lookup failure.

Per report from Peter Geoghegan

Branch
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master

Details
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http://git.postgresql.org/pg/commitdiff/5db55c6bbca28b8be4d68d2fa0e22465cfe8e755

Modified Files
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doc/src/sgml/libpq.sgml      |    3 ++-
src/backend/utils/mb/wchar.c |   19 +++++--------------
2 files changed, 7 insertions(+), 15 deletions(-)