gcc warning fix: -Wextra

For the "ordered comparison of pointer with integer zero" warning, fix
this by comparing (type) -1 against (type) 0 instead of just 0, so if
"type" is a pointer type, this pointer type will be applied to the right
operand too, thus fixing the warning.

Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>