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)?

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Codecov Report

Attention: Patch coverage is 0% with 2 lines in your changes missing coverage. Please review.

Project coverage is 54.21%. Comparing base (1c59d17) to head (123a997).
Report is 47 commits behind head on develop.

Files with missing lines	Patch %	Lines
bfe_bufio/bufio.go	0.00%	1 Missing and 1 partial ⚠️

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Additional details and impacted files

@@             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     

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