If process_ansi encountered an unknown character when processing an ANSI
escape sequence, it would ungetc all the characters read so far, except
for the character just read, and the opening '\033['.  ungetting the
middle of the escape sequence does not produce useful results, and also
relies on the unportable assumption that ungetc works on multiple
characters (which glibc does not support).  Discard the characters
instead.

Signed-off-by: Josh Triplett <josh@joshtriplett.org>