iproto: fix a crashing bug on ec2.large instance

When entire contents of the output buffer is flushed, we must reset
write start and end positions. If the buffer can be recyced, they
both are reset to point to the beginning of the buffer, otherwise write start
is advanced to match write end.

The old code would always execute *begin = *end assignment, regardless
of whether it's case 1) (full reset) or 2) (advancing write start).

gcc 4.8 could reorder this assignment, and put it in front of conditional
iobuf_reset(). In that case, write start could point beyond write end,
since iobuf_reset() resets write end to zero.

A funny bug which would hit only under heavy load, only if there are
-O2 optimizations, and no debug prints.