Fix crash when renaming a glyph to an existing name

CI_CheckMetaData() checks if the new glyph name is used anywhere else
and prompts the user if he wants to swap glyph names, later we will
crash because the cachedsc of that CharInfo will end up with NULL sf.
Merge pull request #490 from JoesCat/master

Fix crash when renaming a glyph to an existing name