Fix possible crash with config panel when engine switched (#3761)
Fix possible crash with config panel when engine switched (#3761)

Reading an epub file with Mupdf would show 6 items in bottom
config panel. Reading it with crengine would show only 5.
A crash would happen if we were on the 6th when leaving MuPDF,
and later opening config panel with crengine.
mwoz123/koreader,Frenzie/koreader,koreader/koreader,poire-z/koreader,koreader/koreader,lgeek/koreader,NiLuJe/koreader,Frenzie/koreader,pazos/koreader,Markismus/koreader,mihailim/koreader,poire-z/koreader,NiLuJe/koreader,Hzj-jie/koreader,houqp/koreader