[ISSUE #876] Safely checking context in trace producer.

[aireet]

What is the purpose of the change

when i use interceptor for build my apm, i got panic in async producer, so I think it can be safely

close #876

Brief changelog

XX

Verifying this change

XXXX

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[codecov-commenter]

Codecov Report

Merging #863 (f8c1bbd) into master (129701a) will increase coverage by 0.01%.
The diff coverage is 30.76%.

@@            Coverage Diff             @@
##           master     #863      +/-   ##
==========================================
+ Coverage   25.00%   25.01%   +0.01%     
==========================================
  Files          51       51              
  Lines        6632     6640       +8     
==========================================
+ Hits         1658     1661       +3     
- Misses       4750     4755       +5     
  Partials      224      224              

Impacted Files	Coverage Δ
primitive/ctx.go	0.00% <0.00%> (ø)
producer/interceptor.go	0.00% <0.00%> (ø)
producer/producer.go	28.35% <57.14%> (+0.35%)	⬆️

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[ShannonDing]

LGTM