Add nil check for controller when agent is shutting down

[dhurley]

Proposed changes

Add nil check for controller when agent is shutting down to prevent the follow error from happening.

panic: runtime error: invalid memory address or nil pointer dereference
[signal SIGSEGV: segmentation violation code=0x1 addr=0x18 pc=0x10087d12a]

goroutine 45 [running]:
main.handleSignals.func1()
        /Users/d.hurley/go/src/github.com/nginx/agent/main.go:141 +0x2ca
created by main.handleSignals
        /Users/d.hurley/go/src/github.com/nginx/agent/main.go:126 +0x1f8
exit status 2
make: *** [run] Error 1

Checklist

Before creating a PR, run through this checklist and mark each as complete.

• I have read the CONTRIBUTING document
• I have run make install-tools and have attached any dependency changes to this pull request
• If applicable, I have added tests that prove my fix is effective or that my feature works
• If applicable, I have checked that any relevant tests pass after adding my changes
• If applicable, I have updated any relevant documentation (README.md)
• If applicable, I have tested my cross-platform changes on Ubuntu 22, Redhat 8, SUSE 15 and FreeBSD 13

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[codecov-commenter]

Codecov Report

Patch coverage has no change and project coverage change: -0.15 ⚠️

Comparison is base (4716d2a) 67.31% compared to head (8d0331a) 67.16%.

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Additional details and impacted files

@@            Coverage Diff             @@
##             main     #357      +/-   ##
==========================================
- Coverage   67.31%   67.16%   -0.15%     
==========================================
  Files         113      113              
  Lines       12731    12738       +7     
==========================================
- Hits         8570     8556      -14     
- Misses       3588     3609      +21     
  Partials      573      573              

see 7 files with indirect coverage changes

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