base64.c: Don't out-of-bounds printf("%s", &encoded[-1]).

`cp' is walking through encoded[] when an error occurs and is stepped
back up to 20 elements to provide some lead-in context for the error
message.  If might be stepped back to encoded-1, but it attempts to cope
with that by `cp ? cp : encoded'.  cp is always non-NULL so true and cp
is printed.  Presumably, `cp > encoded' was meant.  But it's all a bit
of a rigmarole so just use min() instead to ensure cp stays within
encoded and print cp.  Fixes bfc6b93af.