lib-mail: ostream-dot - Fix potential assert-crash when parent stream buffer gets...

lib-mail: ostream-dot - Fix potential assert-crash when parent stream buffer gets full

If max_bytes=1, the (max_bytes-2) calculation brings it to (size_t)-1. This
causes too much data to be sent to the parent stream, which then returns a
partial write and causes an assert-crash.

The final chunk calculation doesn't need the -2 check, because additional
bytes aren't inserted at that point.

Fixes:
Panic: file ostream-dot.c: line 208 (o_stream_dot_sendv): assertion failed: ((size_t)ret == sent + added)