fetchmail -- heap overflow on verbose X.509 display

Matthias Andree reports:

In verbose mode, fetchmail prints X.509 certificate subject and
issuer information to the user, and counts and allocates a malloc()
buffer for that purpose.
If the material to be displayed contains characters with high bit
set and the platform treats the “char” type as signed, this can cause
a heap buffer overrun because non-printing characters are escaped as
\xFF…FFnn, where nn is 80…FF in hex.