Prevent panic in stopwatch

[zeripath]

There is a panic in the stopwatch API whereby prepareIssueStopwatch will return a nil issue causing panics after reporting a http status code.

Signed-off-by: Andrew Thornton art27@cantab.net

[codecov-io]

Codecov Report

Merging #10670 into master will decrease coverage by 0.02%.
The diff coverage is 50%.

@@            Coverage Diff             @@
##           master   #10670      +/-   ##
==========================================
- Coverage   43.61%   43.59%   -0.03%     
==========================================
  Files         588      588              
  Lines       82485    82487       +2     
==========================================
- Hits        35977    35961      -16     
- Misses      42045    42065      +20     
+ Partials     4463     4461       -2

Impacted Files	Coverage Δ
routers/api/v1/repo/issue_stopwatch.go	81.67% <50%> (+4.95%)	⬆️
models/repo_mirror.go	2.12% <0%> (-10.64%)	⬇️
modules/avatar/avatar.go	48% <0%> (-6%)	⬇️
services/pull/check.go	50% <0%> (-4.27%)	⬇️
modules/notification/indexer/indexer.go	50.66% <0%> (-4%)	⬇️
modules/git/command.go	86.95% <0%> (-2.61%)	⬇️
models/unit.go	37.03% <0%> (-2.47%)	⬇️
modules/indexer/issues/indexer.go	56.19% <0%> (-1.33%)	⬇️
modules/log/event.go	64.61% <0%> (-1.03%)	⬇️
routers/repo/view.go	36.14% <0%> (-0.68%)	⬇️
... and 6 more

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