knownhost.c: fix use of uninitialized argument variable wrote

Detected by clang scan in	line 1195, column 18.

src/knownhost.c

@@ -1185,7 +1185,7 @@ libssh2_knownhost_writefile(LIBSSH2_KNOWNHOSTS *hosts,
     for(node = _libssh2_list_first(&hosts->head);
         node;
         node = _libssh2_list_next(&node->node)) {
-        size_t wrote;
+        size_t wrote = 0;
         size_t nwrote;
         rc = knownhost_writeline(hosts, node, buffer, sizeof(buffer), &wrote,
                                  type);