tools: gpio-hammer: Avoid potential overflow in main

If '-o' was used more than 64 times in a single invocation of gpio-hammer,
this could lead to an overflow of the 'lines' array. This commit fixes
this by avoiding the overflow and giving a proper diagnostic back to the
user

Signed-off-by: Gabriel Ravier <gabravier@gmail.com>
Signed-off-by: Bartosz Golaszewski <bgolaszewski@baylibre.com>