app_voicemail: Fix json blob errors

When app_voicemail calls ast_test_suite_notify with the results of
a user keypress, it formats the keypress as '%c'.  If the user hung up
or some other error occurrs, the result of the keypress is a non
printable character.  This ultimately causes json_vpack_ex to think
it's being passed a non utf-8 string and return an error.

* Keypress results passed to ast_test_suite_notify are now checked with
  isprint() and a '?' is substituted if the check fails.

Change-Id: I78ee188916bbac840f3d03f40201b692347ea865