shared: end string with % if one was found at the end of a expandible string (#6828)

Current behavior is that %X where X is an unidentified specifier, then the result is
the same %X string. This was not the case when the string ended with a stray %, where
the character would have not been output. Lets add that missing character.

Fixes: #6374
(cherry picked from commit 038492ae)