Snapshot continued NullPointerException

[dominicqi]

Please answer these questions before submitting your issue.

• Why do you submit this issue?
• Question or discussion
• [-] Bug
• Requirement
• Feature or performance improvement

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Question

• What do you want to know?

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Bug

• Which version of SkyWalking, OS and JRE?

• Which company or project?

• What happen?

ContextManager#continued remove valid about IgnoredTraceContext.

https://github.com/apache/skywalking/blob/master/apm-sniffer/apm-agent-core/src/main/java/org/apache/skywalking/apm/agent/core/context/ContextManager.java#L143

Here will throw NullPointerException if the pre-TraceContext is IgnoredContext.Because traceSegmentId is null.

https://github.com/apache/skywalking/blob/master/apm-sniffer/apm-agent-core/src/main/java/org/apache/skywalking/apm/agent/core/context/ContextSnapshot.java#L53

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Requirement or improvement

• Please describe about your requirements or improvement suggestions.
  should allow IgnoredTraceContext continue

[wu-sheng]

@mrproliu please take a look.