Refactor check of http Response.ContentLength

[linux019]

The check resp.ContentLength <= maxBodySlurpSize supersedes resp.ContentLength == -1 as ContentLength is signed int64
https://play.golang.org/p/Io9U59wxxkK

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[codecov]

Codecov Report

Merging #1615 into master will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master    #1615   +/-   ##
=======================================
  Coverage   68.04%   68.04%           
=======================================
  Files          97       97           
  Lines        8830     8830           
=======================================
  Hits         6008     6008           
  Misses       1908     1908           
  Partials      914      914           

Impacted Files	Coverage Δ
github/github.go	89.93% <100.00%> (ø)

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[gmlewis]

How about this instead?

Suggested change

	if resp.ContentLength <= maxBodySlurpSize {
	if resp.ContentLength != 0 && resp.ContentLength <= maxBodySlurpSize {

[gmlewis]

And while you are making tweaks, how about wrapping lines 572-576 with a check like:

if resp != nil {
...
}

[itsksaurabh]

@gmlewis @linux019
The value -1 indicates that the length is unknown.

 // ContentLength records the length of the associated content.
    // The value -1 indicates that the length is unknown.
    // Values >= 0 indicate that the given number of bytes may
    // be read from Body.
    //
    // For client requests, a value of 0 with a non-nil Body is
    // also treated as unknown.
    ContentLength int64

Please check it here. https://golang.org/pkg/net/http/

[gmlewis]

Closing this PR in favor of #1645.