The DMG creation step sets DMG_TMP from 'mktemp -d' with no error check.
If mktemp fails (tmpfs full, permissions, TMPDIR misconfigured), DMG_TMP
is empty and the very next line — 'cp -a "\$APP_DIR" "\$DMG_TMP/"' —
expands to 'cp -a "<app>" "/"', which copies the bundle into the root of
the filesystem.
Refuse to continue unless mktemp produced a real directory. Defensive
second check catches the (rare) case where mktemp succeeds but returns
something that isn't a directory we can cp into.
