Sentinel: Skip IS-PRIMARY-DOWN-BY-ADDR requests when primary not SDOWN

[zhijun42]

A super tiny change to optimize the function sentinelAskPrimaryStateToOtherSentinels to early return when the sentinel does not observe the primary as subjectively down.

I ran ./runtest-sentinel locally and saw no breaking change.

[codecov]

Codecov Report

❌ Patch coverage is 0% with 4 lines in your changes missing coverage. Please review.
✅ Project coverage is 72.20%. Comparing base (8d562d2) to head (7f25d71).

Files with missing lines	Patch %	Lines
src/sentinel.c	0.00%	4 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##           unstable    #2633      +/-   ##
============================================
- Coverage     72.22%   72.20%   -0.02%     
============================================
  Files           127      127              
  Lines         70936    70934       -2     
============================================
- Hits          51232    51219      -13     
- Misses        19704    19715      +11     

Files with missing lines	Coverage Δ
src/sentinel.c	0.00% <0.00%> (ø)

... and 13 files with indirect coverage changes

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