Fix MimeMessage::extract_uue() ignoring index greater than zero

[iliaal]

What

MimeMessage::extract_uue($index, ...) returns NULL for any $index >= 1, so only the first uuencoded attachment in a message is reachable.

The method compares a part counter nparts against $index but never increments it, unlike the sibling enum_uue which does. So nparts == index only ever matches index 0.

Fix

Increment nparts after skipping a non-matching part, so the counter advances to the requested index.

[iliaal]

Superseded by #73, reopened from the fork (iliaal/pecl-mail-mailparse). Branch should not have been pushed to php/ directly.