net/tcpview: Permanently fix LLVM15 errors

Fix int-conversion errors flagged by LLVM15.

detail-domain.c:573:13: error: incompatible pointer to integer conversion
returning 'void *' from a function with result type 'int' [-Wint-conversion]
            return NULL;
                   ^~~~
/usr/include/sys/_null.h:34:14: note: expanded from macro 'NULL'
                ^~~~~~~~~~~