Fix inappropriate uses of PG_GETARG_UINT32()

Fix inappropriate uses of PG_GETARG_UINT32()

The chr() function used PG_GETARG_UINT32() even though the argument is
declared as (signed) integer.  As a result, you can pass negative
arguments to this function and it internally interprets them as
positive.  Ultimately ends up being harmless, but it seems wrong, so
fix this and rearrange the internal error checking a bit to
accommodate this.

Another case was in the documentation, where example code used
PG_GETARG_UINT32() with an argument declared as signed integer.

Reviewed-by: Nathan Bossart 
Discussion: https://www.postgresql.org/message-id/flat/7e43869b-d412-8f81-30a3-809783edc9a3%40enterprisedb.com