fix: fix miss makezero bug

[alingse]

I was running github actions to run linter makezero for top github golang repos.

see issues alingse/go-linter-runner#1

and the github actions output https://github.com/alingse/go-linter-runner/actions/runs/9241873083/job/25423942721

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append to slice `nodes` with non-zero initialized length at https://github.com/google/pprof/blob/main/internal/graph/graph.go#L44[9](https://github.com/alingse/go-linter-runner/actions/runs/9241873083/job/25423942721#step:4:10):11
====================================================================================================

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[codecov-commenter]

Codecov Report

Attention: Patch coverage is 0% with 1 lines in your changes are missing coverage. Please review.

Project coverage is 66.98%. Comparing base (0ed6a68) to head (a699076).
Report is 24 commits behind head on main.

Files	Patch %	Lines
internal/graph/graph.go	0.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #867      +/-   ##
==========================================
+ Coverage   66.86%   66.98%   +0.11%     
==========================================
  Files          44       44              
  Lines        9824     9793      -31     
==========================================
- Hits         6569     6560       -9     
+ Misses       2794     2780      -14     
+ Partials      461      453       -8     

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