libmisc/yesno.c: Fix regression

libmisc/yesno.c: Fix regression

The getline function does not return a pointer but the amount of read
characters. The error return value to check for is -1.

Set buf to NULL to avoid dereference of an uninitialized stack value.

The getline function returns -1 if size argument is NULL. Always use
a valid pointer even if size is unimportant.

Signed-off-by: Samanta Navarro <ferivoz@riseup.net>