scripts/kconfig/nconf: dynamically alloc dialog_input_result

To support unlimited length string config items;

No check for realloc return value keeps code simple, and to be
consistent with other existing unchecked malloc in kconfig.

Signed-off-by: Cheng Renquan <crquan@gmail.com>
Signed-off-by: Arnaud Lacombe <lacombar@gmail.com>