avoid 'the address of ... will always evaluate as ...' warns in mem macros

avoid  'the address of ... will always evaluate as ...' warns in mem macros

In f14cf363205 we added asserts to our memory macros (Copy(), Zero() etc)
to ensure that the target is non-null. These asserts throw warnings like

    perl.c: In function ‘Perl_eval_sv’:
    perl.c:2976:264: warning: the address of ‘myop’ will always evaluate
    as ‘true’ [-Waddress]
         Zero(&myop, 1, UNOP);

which is annoying. This patch changes how these asserts are coded so
we avoid the warning. Thanks to Zefram for the fix.