Bug#935724: aegisub: Aegisub crashes immediately after opening the context menu

[wangqr]

Looks like the crashing Aegisub has been build without hunspell (it is 
not in the depends list). It will crash due to using a NULL pointer to 
the non-existent spell checker. See 
https://github.com/wangqr/Aegisub/commit/0b8b286767bf796ecfc993c38c4ad14091703162#diff-64304e039535719d12a4a36d333a555c