kconfig: fix potential NULL pointer dereference in conf_askvalue

kconfig: fix potential NULL pointer dereference in conf_askvalue

In conf_askvalue(), the 'def' argument (retrieved via sym_get_string_value)
can be NULL. While current call sites ensure that 'def' is valid,
calling printf("%s\n", def) is technically undefined behavior and could
lead to a segmentation fault on certain libc implementations if the
function were called with a NULL pointer in the future.

Improve the robustness of conf_askvalue() by providing an empty string
as a fallback.

Additionally, remove the redundant re-initialization of the 'line'
buffer inside the !sym_is_changeable(sym) block, as it is already
properly initialized at the function entry.

Fixes: 1da177e4c3f4 ("Linux-2.6.12-rc2")
Signed-off-by: Xingjing Deng <micro6947@gmail.com>
Reviewed-by: Nathan Chancellor <nathan@kernel.org>
Link: https://patch.msgid.link/20260306021709.27068-1-micro6947@gmail.com
Signed-off-by: Nathan Chancellor <nathan@kernel.org>