lib-sieve: util: edit-mail: Fix assert panic occurring when streaming large headers.

The problem mainly arose because the stream buffer position was used to
determine how much was written, while the stream buffer position is potentially
decreased by i_stream_try_alloc().

Panic was:

Panic: file istream.c: line 197 (i_stream_read): assertion failed: ((size_t)ret+old_size == _stream->pos - _stream->skip)