[meshcop] fix premature free of finalize message (#4487)
[meshcop] fix premature free of finalize message (#4487)

This commit fixes a bug that the finalize message will be prematurely
freed if the joiner is started at the second time without waiting for
the finish of the first initiation.
bukepo/openthread,lanyuwen/openthread,bukepo/openthread,openthread/openthread,chshu/openthread,abtink/openthread,jwhui/openthread,bukepo/openthread,openthread/openthread,librasungirl/openthread,chshu/openthread,lanyuwen/openthread,jwhui/openthread,srickardti/openthread,lanyuwen/openthread,chshu/openthread,lanyuwen/openthread,srickardti/openthread,openthread/openthread,srickardti/openthread,chshu/openthread,abtink/openthread,librasungirl/openthread,librasungirl/openthread,jwhui/openthread,srickardti/openthread,lanyuwen/openthread,jwhui/openthread,openthread/openthread,chshu/openthread,abtink/openthread,abtink/openthread,lanyuwen/openthread,librasungirl/openthread,bukepo/openthread,chshu/openthread