Fix ess-build-tags-for-directory if no TAGS filename provided

Running ess-build-tags-for-directory appears to default to the current directory (e.g. mypackage/R) when asking for the name of the TAGS file. This should default to TAGS within that directory. However if I simply accept the default emacs (version 24.3.1) provides the full file path with the R file being edited currently, causing that file to be overwritten on disk.