Low: tools: Fix overflow in crm_simulate --repeat

Due to the way glib parses integer options and casts them to assign them
to variables, a value of --repeat="-1" wrapped around to 2^32 - 1,
causing a very long loop. Instead, we default it to 0 if a negative
value is passed in.

Signed-off-by: Reid Wahl <nrwahl@protonmail.com>