make sure the buff can be written

[ccqy66]

at the same time, I think the following codes don't make sense:

if (b.w + n) > len(b.buf) {
	log.Logger.Warn("bfe_bufio:reader.fill(),len(buf)=%d,b.r=%d,b.w=%d,n=%d\n",
			len(b.buf), b.r, b.w, n
}

theoretically, b.w+n <= len(b.buf). when b.w+n can be > len(b.buf)?

[codecov-commenter]

Codecov Report

Merging #1057 (123a997) into develop (1c59d17) will decrease coverage by 0.01%. The diff coverage is 0.00%.

@@             Coverage Diff             @@
##           develop    #1057      +/-   ##
===========================================
- Coverage    54.23%   54.21%   -0.02%     
===========================================
  Files          301      301              
  Lines        26604    26606       +2     
===========================================
- Hits         14429    14425       -4     
- Misses       10509    10513       +4     
- Partials      1666     1668       +2     

Impacted Files	Coverage Δ
bfe_bufio/bufio.go	83.74% <0.00%> (-0.47%)	:arrow_down:
...e_modules/mod_trust_clientip/mod_trust_clientip.go	42.85% <0.00%> (-3.30%)	:arrow_down:
bfe_websocket/server_conn.go	74.24% <0.00%> (-2.28%)	:arrow_down:
bfe_http2/writesched.go	79.38% <0.00%> (-1.53%)	:arrow_down:
bfe_spdy/server_conn.go	78.53% <0.00%> (-0.45%)	:arrow_down:
bfe_spdy/frame_write.go	50.50% <0.00%> (ø)
bfe_http2/server.go	72.36% <0.00%> (+0.32%)	:arrow_up:
bfe_spdy/server_write_sched.go	80.00% <0.00%> (+1.53%)	:arrow_up:

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