(tumme-track-original-file): Add `buffer-live-p' check.

(tumme-track-original-file): Add `buffer-live-p' check.
(tumme-format-properties-string): Handle empty `buf'.
(tumme-get-comment): Change variable names inside `let'.  Add
missing `let' variable that cause font-lock problems.
(tumme-write-comments): Change variable names inside `let'.  Add
missing `let' variable that cause font-lock problems.
(tumme-forward-image): Rename from `tumme-forward-char'.
(tumme-backward-image): Rename from `tumme-backward-char'.