Fix errors due to bad strncpy behavour.

As of GCC 8.1 a compiler error (stringop-truncation) will be raised as
strncpy has potentially unsafe behavour, which is the case here.

If strncpy's source (path in our case) is longer than the destination
the destination will not be null terminated. Which is what sockaddr_un
requires of sockaddr_un.sun_path.

By reducing the copy size by one ensures as much as path as can be
copied is and pads the rest of destination (the remaining one
character) with null bytes, ensuring it's properly null terminated.

src/apps/vhost/vhost_user.c

@@ -33,7 +33,7 @@ int vhost_user_connect(const char *path)
     }
 
     un.sun_family = AF_UNIX;
-    strncpy(un.sun_path, path, sizeof(un.sun_path));
+    strncpy(un.sun_path, path, sizeof(un.sun_path)-1);
 
     if (connect(sock, (struct sockaddr *) &un, sizeof(un)) == -1) {
         close(sock);
@@ -54,7 +54,7 @@ int vhost_user_listen(const char *path)
     }
 
     un.sun_family = AF_UNIX;
-    strncpy(un.sun_path, path, sizeof(un.sun_path));
+    strncpy(un.sun_path, path, sizeof(un.sun_path)-1);
     unlink(un.sun_path);
     if (bind(sock, (struct sockaddr *) &un, sizeof(un)) == -1) {
         close(sock);