menuconfig: Fix memory leak introduced by jump keys feature

Fixes the memory leak of struct jump_key allocated in get_prompt_str()

Signed-off-by: Benjamin Poirier <bpoirier@suse.de>
Tested-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Reviewed-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Cc: stable@vger.kernel.org