fix(ddtrace/tracer): fix panic in SQLCommentCarrier

[rarguelloF]

What does this PR do?

Fixes #2837

Motivation

Reviewer's Checklist

• Changed code has unit tests for its functionality at or near 100% coverage.
• System-Tests covering this feature have been added and enabled with the va.b.c-dev version tag.
• There is a benchmark for any new code, or changes to existing code.
• If this interacts with the agent in a new way, a system test has been added.
• Add an appropriate team label so this PR gets put in the right place for the release notes.
• Non-trivial go.mod changes, e.g. adding new modules, are reviewed by @DataDog/dd-trace-go-guild.
• For internal contributors, a matching PR should be created to the v2-dev branch and reviewed by @DataDog/apm-go.

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[datadog-datadog-prod-us1]

Datadog Report

Branch report: rarguelloF/fix-sqlcomment-panic
Commit report: b56aa38
Test service: dd-trace-go

✅ 0 Failed, 5122 Passed, 70 Skipped, 2m 54.58s Total Time
❄️ 1 New Flaky

New Flaky Tests (1)

• TestExchangeContext - gopkg.in/DataDog/dd-trace-go.v1/contrib/miekg/dns - Last Failure

  Expand for error

   Failed
   
   === RUN   TestExchangeContext
       dns_test.go:47: listen udp 127.0.0.1:2020: bind: address already in use
       dns_test.go:85: 
           	Error Trace:	/home/runner/work/dd-trace-go/dd-trace-go/contrib/miekg/dns/dns_test.go:85
           	Error:      	Received unexpected error:
           	            	read udp 127.0.0.1:53660->127.0.0.1:2020: read: connection refused
           	Test:       	TestExchangeContext
       dns_test.go:53: 
   ...