Use clear-string instead of fillarray to clobber secret strings

Use clear-string instead of fillarray to clobber secret strings

* lisp/net/sasl-cram.el (sasl-cram-md5-response):
* lisp/net/sasl-digest.el (sasl-digest-md5-response-value):
* lisp/net/sasl.el (sasl-plain-response):
`fillarray` signals an error for strings that contain multibyte chars;
`clear-string` always works for this purpose.

(cherry picked from commit 1ac70626fa6720a407a23b1b9c14364e5a9230ae)