BUG/MEDIUM: peers: make "show peers" more careful about partial initialization

BUG/MEDIUM: peers: make "show peers" more careful about partial initialization

Since 2.6 with commit 34e4085f8 ("MEDIUM: peers: Balance applets across
threads") the initialization of a peers appctx may be postponed with a
wakeup, causing some partially initialized appctx to be visible. The
"show peers" command used to only care about peers without appctx, but
now it must also take care of those with no stconn, otherwise it can
occasionally crash while dumping them.

This fix must be backported to 2.6. Thanks to Patrick Hemmer for
reporting the problem.