src/utils.c: Undo a while loop optimisation in process_mime()

Change the while loop condition from "bytes_read == BUF_SIZE" to
"bytes_read > 0". This is the better paradigm to use as the kernel may
actually give you less data then you requested while still having more
to send. This czn happen with pipes.

Signed-off-by: Andrew Clayton <andrew@dnsandmx.com>