Initialize jump variable

This remove the error given when running 'make config':

In file included from scripts/kconfig/zconf.tab.c:2503:0:
scripts/kconfig/menu.c: In function 'get_symbol_str':
scripts/kconfig/menu.c:567:18: warning: 'jump' may be used uninitialized in this function [-Wmaybe-uninitialized]
     jump->offset = r->len - 1;
                  ^
scripts/kconfig/menu.c:528:19: note: 'jump' was declared here
  struct jump_key *jump;