smb: use memcpy() instead of strncpy()

... as it never copies the trailing zero anyway and always just the four
bytes so let's not mislead anyone into thinking it is actually treated
as a string.

Coverity CID: 1260214

lib/smb.c

@@ -345,7 +345,7 @@ static void smb_format_message(struct connectdata *conn, struct smb_header *h,
   memset(h, 0, sizeof(*h));
   h->nbt_length = htons((unsigned short) (sizeof(*h) - sizeof(unsigned int) +
                                           len));
-  strncpy((char *)h->magic, "\xffSMB", 4);
+  memcpy((char *)h->magic, "\xffSMB", 4);
   h->command = cmd;
   h->flags = SMB_FLAGS_CANONICAL_PATHNAMES | SMB_FLAGS_CASELESS_PATHNAMES;
   h->flags2 = smb_swap16(SMB_FLAGS2_IS_LONG_NAME | SMB_FLAGS2_KNOWS_LONG_NAME);