[perl #120374] Stop for($h{k}||'') from vivifying

[perl #120374] Stop for($h{k}||'') from vivifying

Commit 2e73d70e52 broke this (made it vivify) by propagating lvalue
context to the branches of || and && (to fix another bug).  It broke
App::JobLog as a result.

Because foreach does not do defelem magic (i.e., it vivifies), this
ends up extending vivification to happen where it did not before.

Fixing foreach to do defelem magic (create ‘deferred element’ scalars,
the way sub calls do, to avoid vivifying immediately) would be another
way to fix this, but it is controversial.  See ticket #2166.

So, if either argument to || (or &&) is a vivifying op, don’t propa-
gate the lvalue context, unless this is the return value of an lvalue
sub (necessary for if/else with implicit return to work correctly in
lvalue subs).