Fix NegotiateFormat

[eleven26]

Fix issue #3241

When the length of the offered mime type string is inconsistent with the request's accepted mime length, it may cause a panic, but it should actually return an empty string.

[codecov[bot]]

Codecov Report

Merging #3243 (b593c6e) into master (b57163a) will increase coverage by 0.00%. The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master    #3243   +/-   ##
=======================================
  Coverage   98.37%   98.38%           
=======================================
  Files          43       43           
  Lines        3148     3149    +1     
=======================================
+ Hits         3097     3098    +1     
  Misses         38       38           
  Partials       13       13           

Flag	Coverage Δ
go-1.15	98.38% <100.00%> (+<0.01%)	:arrow_up:
go-1.16	98.35% <100.00%> (+<0.01%)	:arrow_up:
go-1.17	98.28% <100.00%> (+<0.01%)	:arrow_up:
go-1.18	98.28% <100.00%> (+<0.01%)	:arrow_up:
macos-latest	98.38% <100.00%> (+<0.01%)	:arrow_up:
nomsgpack	98.35% <100.00%> (+<0.01%)	:arrow_up:
ubuntu-latest	98.38% <100.00%> (+<0.01%)	:arrow_up:

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
context.go	98.46% <100.00%> (+<0.01%)	:arrow_up:

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[thinkerou]

please also review the pull request @dawkaka thanks!