syscall: fix use of uninitialized values

linux-user/syscall.c:1627:35: warning: 1st function call argument is an uninitialized value
        target_saddr->sa_family = tswap16(addr->sa_family);
                                  ^~~~~~~~~~~~~~~~~~~~~~~~
linux-user/syscall.c:1629:25: warning: The left operand of '==' is a garbage value
    if (addr->sa_family == AF_NETLINK && len >= sizeof(struct sockaddr_nl)) {
        ~~~~~~~~~~~~~~~ ^

Reported-by: Clang Static Analyzer
Signed-off-by: Philippe Mathieu-Daudé <[email protected]>
Reviewed-by: Laurent Vivier <[email protected]>
Signed-off-by: Michael Tokarev <[email protected]>

linux-user/syscall.c

@@ -1622,6 +1622,7 @@ static inline abi_long host_to_target_sockaddr(abi_ulong target_addr,
     if (len == 0) {
         return 0;
     }
+    assert(addr);
 
     target_saddr = lock_user(VERIFY_WRITE, target_addr, len, 0);
     if (!target_saddr)