Handle failed psync log when there is no replication backlog

[enjoy-binbin]

This crash was introduced in #2877, we will crash when there is no replication backlog.

[enjoy-binbin]

Sorry for this mistake, CI reported the crash before, but I didn't notice it at the time.

[codecov]

Codecov Report

❌ Patch coverage is 66.66667% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 72.45%. Comparing base (a087cc1) to head (9baff20).
⚠️ Report is 1 commits behind head on unstable.

Files with missing lines	Patch %	Lines
src/replication.c	66.66%	1 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##           unstable    #2886      +/-   ##
============================================
+ Coverage     72.43%   72.45%   +0.02%     
============================================
  Files           128      128              
  Lines         70485    70487       +2     
============================================
+ Hits          51055    51072      +17     
+ Misses        19430    19415      -15     

Files with missing lines	Coverage Δ
src/replication.c	85.94% <66.66%> (+0.04%)	⬆️

... and 10 files with indirect coverage changes

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[sarthakaggarwal97]

This is where Merge Queue (#2820) would have helped as well.