Avoid E341 (Internal error: lalloc(0, )).
The E341 was happening with filetypes that include R (Rnoweb, Rmd, Rrst,
Rhelp, and Rdoc) if the user had quickly sent to R two commands that
load libraries. The second command sent to R would make the Vim-R-plugin
run the Vim filetype command again before the first one has been
processed.