Fix ietf-drums-remove-whitespace unmatched " and (

Fix ietf-drums-remove-whitespace unmatched " and (

* lisp/mail/ietf-drums.el:
+ (ietf-drums-skip-comment):  New helper function.
+ (ietf-drums-remove-comments):  Use ietf-drums-skip-comment.
+ (ietf-drums-remove-whitespace):  Handle unterminated quotes and
comments, as ietf-drums-remove-comments already does.
* test/lisp/mail/ietf-drums-tests.el:
+ Test unterminated quote and comment for
ietf-drums-remove-whitespace and ietf-drums-remove-comments (bug#53965).