cl_delete_corpus() crashes if corpus has not been loaded before

[ablaette]

The cl_delete_corpus() function works as intended if a corpus has been loaded before:

library(RcppCWB)
cl_attribute_size(corpus = "UNGA", attribute = "word", attribute_type = "p")
cl_delete_corpus("UNGA")

But there is a nasty crash if the corpus is not loaded - e.g. if it has been deleted before.

library(RcppCWB)
cl_delete_corpus("UNGA")
cl_delete_corpus("UNGA")

[ablaette]

The Rcpp/C wrapper behind cl_delete_corpus() now checks whether a corpus is loaded and does not attempt the unload if not.