Terminate filter_create_ext() args with NULL, not 0.

filter_create_ext() is documented to take a NULL terminated set of
arguments. 0 is promoted to an int so this would fail on 64-bit
systems if the value was not passed in a register. On all currently
supported 64-bit architectures it is.

Obtained from: CheriBSD
Sponsored by: DARPA, AFRL