[router-table] check num active routers before allocating (#2723)
[router-table] check num active routers before allocating (#2723)

Credit to OSS-Fuzz.
srickardti/openthread,jwhui/openthread,librasungirl/openthread,abtink/openthread,lanyuwen/openthread,abtink/openthread,LeZhang2016/openthread,librasungirl/openthread,gandreello/openthread,bukepo/openthread,lanyuwen/openthread,bukepo/openthread,mszczodrak/openthread,mszczodrak/openthread,chshu/openthread,openthread/openthread,abtink/openthread,chshu/openthread,turon/openthread,gandreello/openthread,georgecpr/openthread,librasungirl/openthread,mszczodrak/openthread,georgecpr/openthread,chshu/openthread,erja-gp/openthread,erja-gp/openthread,LeZhang2016/openthread,georgecpr/openthread,lanyuwen/openthread,mszczodrak/openthread,mszczodrak/openthread,jwhui/openthread,chshu/openthread,turon/openthread,LeZhang2016/openthread,LeZhang2016/openthread,georgecpr/openthread,gandreello/openthread,lanyuwen/openthread,gandreello/openthread,LeZhang2016/openthread,abtink/openthread,turon/openthread,srickardti/openthread,jwhui/openthread,georgecpr/openthread,chshu/openthread,mszczodrak/openthread,erja-gp/openthread,openthread/openthread,srickardti/openthread,bukepo/openthread,georgecpr/openthread,erja-gp/openthread,turon/openthread,openthread/openthread,gandreello/openthread,gandreello/openthread,LeZhang2016/openthread,erja-gp/openthread,bukepo/openthread,jwhui/openthread,chshu/openthread,srickardti/openthread,lanyuwen/openthread,lanyuwen/openthread,erja-gp/openthread,librasungirl/openthread,openthread/openthread