Applied Eric Schnoebelen <eric@cirr.com>'s mhshowsbr.c patch fixing apparent

Applied Eric Schnoebelen <eric@cirr.com>'s mhshowsbr.c patch fixing apparent
bugs in Dan Winship's new security quoting code:

    Since upgrading, I've been getting the following errors while attempting to
    process some MIME messages:

        (1) Syntax error: Unterminated quoted string
            exit 2

    and:

        (2) line 1/10 (END)Segmentation fault (core dumped)

    (2) appears to be due to the testing of an unset pointer in
    mhshowsbr.c:show_multi_aux().  (1) appears to be caused by mis-quoting a
    filename being handed to the shell in mhshowsbr.c:show_content_aux().

    Resolving the pointer reference issue in mhshowsbr.c:show_multi_aux() turned
    up a similar mis-quoting problem in the routine.