netfilter: bridge: stp fix reference to uninitialized data

The destination mac (destmac) is only valid if EBT_DESTMAC flag
is set. Fix by changing the order of the comparison to look for
the flag first.

Reported-by:  <syzbot+5c06e318fc558cc27823@syzkaller.appspotmail.com>
Signed-off-by: Stephen Hemminger <stephen@networkplumber.org>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>