regress_ssl: fix ssl/bufferevent_wm_filter for non defered callbacks

regress_ssl: fix ssl/bufferevent_wm_filter for non defered callbacks

Even after referenced patch there is still possible recursive callbacks
from evbuffer_drain(bev_input), i.e.:
  wm_transfer() -> evbuffer_drain() -> wm_transfer()
                   inc(ctx->get)

But if we will increment ctx->get before drain that we will not add more
data to buffer.

Refs: 54c6fe3c ("regress_ssl: make ssl/bufferevent_wm_filter more fault-tolerance")
CI: https://ci.appveyor.com/project/nmathewson/libevent/build/job/f0rv299i71wnuxdq#L2546