lib-sieve: util: edit-mail: istream: Fix bug in calculation of stream position.

This caused an assert panic when the application or child stream did not skip
all buffered data immediately. This was reproducible only when the message input
stream consisted of a concatenation of smaller streams, which is used in LMTP to
prepend a few headers.

The panic was:

Panic: file istream.c: line 329 (i_stream_read_memarea): assertion failed: ((size_t)ret+old_size == _stream->pos - _stream->skip)