The function for retrieving a shape key by its index is named
somewhat confusingly, and effectively reimplements BLI_findlink.
However, more importantly, for some reason it is coded to return
null for the index 0 instead of the basis shape key. This severely
limits its usability in some cases.
This refactor replaces the function with a simple strongly typed
wrapper around BLI_findlink, using a different name, and updating
the call sites to check that the index is not 0 where necessary.
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