exact-address: broken for "group:;" syntax

[Claus Assmann]

A message with headers like this:

----------------------------------------
To: Group:;
Cc: Named: user1;
----------------------------------------

is displayed in mutt as follows when compiled 

without --enable-exact-address:
----------------------------------------
To: Group: ;
Cc: Named:  user1;
----------------------------------------

with --enable-exact-address:
----------------------------------------
To: Group:
Cc: Named: user1
----------------------------------------
that is, the trailing ';' is missing.


This has been verified with Mutt 1.9.0 (2017-09-02), it happens
with earlier versions too.

It would be nice if someone who understands the address parsing
code could fix it :-)

Thanks!