What is the purpose of the change

The line unzip -o ${fname_libevent} >unziplibevent.txt 2>& creates the file unziplibevent.txt in the directory. So when setting the directory name with libevent_dir=$(ls | grep libevent | grep .*[^zip]$) the variable will have the name of two files and cd ${libevent_dir} will prompt cd: Too many arguments.

Brief changelog

Just adding the .txt extension to the exclusion list during the grep command is enough to solve the problem. In the end we have libevent_dir=$(ls | grep libevent | grep .*[^zip^txt]$).