fix: correct syntax error when detecting old ubuntu releases

quickget

@@ -1601,10 +1601,13 @@ function make_vm_config() {
         ubuntu*)
             GUEST="linux"
             IMAGE_TYPE="iso"
-            local SHORT_RELEASE="$(echo "${RELEASE}" | sed s'/\.//g')"
-            if [ ${SHORT_RELEASE} -lt 1604 ]; then
+            # If there is a point in the release, check if it is less than 16.04
+            if [[ "${RELEASE}" == "*.*" ]]; then
+                local SHORT_RELEASE=${RELEASE//./}
+                if [ "${SHORT_RELEASE}" -lt 1604 ]; then
                     GUEST="linux_old"
                 fi
+            fi
             ;;
         windows)
             GUEST="windows"