in6_mc_get: Fix recursion on if_addr_lock on malloc failure

Analogously to r291040, in6_mc_get recurses on if_addr_lock if the
M_NOWAIT allocation fails. The fix is the same.

Suggested by: Andrey V. Elsukov
Reviewed by: jhb (ip4 version)
Sponsored by: EMC / Isilon Storage Division
Differential Revision: https://reviews.freebsd.org/D4138 (ip4 version)