detect_os.sh: fix minor error if os can't be detected.

snippets/detect_os.sh

@@ -64,12 +64,13 @@ detect_os_2 ()
       fi
 
     else
-      unknown_os
+      os_dist="unknown"
+      os="unknown"
     fi
   fi
 
   if [ -z "$os_dist" ]; then
-    unknown_os
+    os_dist="unknown"
   fi
 
   # remove whitespace from OS and os_dist name
@@ -90,6 +91,7 @@ if [[ -n "$os_codename" ]]; then
    echo "Codename:     ${os_codename}"
 fi
 
+os=""
 os_dist=""
 os_version=""
 os_codename=""