[SCTP]: Fix kernel panic while received AUTH chunk with BAD shared key identifier

If SCTP-AUTH is enabled, received AUTH chunk with BAD shared key 
identifier will cause kernel panic.

Test as following:
step1: enabled /proc/sys/net/sctp/auth_enable
step 2:  connect  to SCTP server with auth capable. Association is 
established between endpoints. Then send a AUTH chunk with a bad 
shareid, SCTP server will kernel panic after received that AUTH chunk.

SCTP client                   SCTP server
  INIT         ---------->  
    (with auth capable)
               <----------    INIT-ACK
                              (with auth capable)
  COOKIE-ECHO  ---------->
               <----------    COOKIE-ACK
  AUTH         ---------->


AUTH chunk is like this:
  AUTH chunk
    Chunk type: AUTH (15)
    Chunk flags: 0x00
    Chunk length: 28
    Shared key identifier: 10
    HMAC identifier: SHA-1 (1)
    HMAC: 0000000000000000000000000000000000000000

The assignment of NULL to key can safely be removed, since key_...