coverage.sh: only run cover if coverage data exists

This is useful if ./coverage.sh is run with CMD=./mmdebstrap and thus
gathering coverage data is disabled.

coverage.sh

@@ -597,6 +597,7 @@ done
 guestfish add-ro shared/cover_db.img : run : mount /dev/sda / : tar-out / - \
        | tar -C shared/cover_db --extract
 
+if [ -e shared/cover_db/runs ]; then
 	cover -nogcov -report html_basic shared/cover_db
 	mkdir -p report
 	for f in common.js coverage.html cover.css css.js mmdebstrap--branch.html mmdebstrap--condition.html mmdebstrap.html mmdebstrap--subroutine.html standardista-table-sorting.js; do
@@ -607,5 +608,6 @@ cover -delete shared/cover_db
 	echo
 	echo open file://$(pwd)/report/coverage.html in a browser
 	echo
+fi
 
 rm shared/tar1.txt shared/tar2.txt