fix: fix ignored selection key setting when candidate per page is less than 10

[kanru]

libchewing API document says

selkeys is an ASCII code integer array of length MAX_SELKEY. The second argument is unused.

However, it actually resets the selkey to default if any length that is not 10 is passed. Fixing the API is more dangerous because we might accidentally read pass end of array passed in. So fixing in ibus-chewing instead.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 52.29%. Comparing base (208867a) to head (7b1652e).
Report is 2 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #247   +/-   ##
=======================================
  Coverage   52.29%   52.29%           
=======================================
  Files          17       17           
  Lines        2096     2096           
=======================================
  Hits         1096     1096           
  Misses       1000     1000           

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