Fix issues with --skip-extensions flag (#587)

There were few issues with --skip-extensions flag

1) Excluded extensions were not part of the filter
2) Invalid memory access from pg_dump_db_extension_namespace_hook
3) Filter table populated without checking whether it is already done or
   not.

Signed-off-by: Arunprasad Rajkumar <ar.arunprasad@gmail.com>