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 <f4bug@amsat.org>
Reviewed-by: Laurent Vivier <laurent@vivier.eu>
Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>
diff --git a/linux-user/syscall.c b/linux-user/syscall.c
index 71d45a9..81f52f7 100644
--- a/linux-user/syscall.c
+++ b/linux-user/syscall.c
@@ -1622,6 +1622,7 @@
     if (len == 0) {
         return 0;
     }
+    assert(addr);
 
     target_saddr = lock_user(VERIFY_WRITE, target_addr, len, 0);
     if (!target_saddr)