nsxiv/a9d2b5b

[ci]: suppress some cppcheck warnings

cppcheck warns about passing a plain NULL to construct_argv()

	main.c:254:36: portability: Passing NULL after the last typed argument to a variadic function leads to undefined behaviour. [varFuncNullUB]
	                fidx, fcnt, w, h, z, NULL);

in our case, POSIX mandates `NULL` to be defined as `(void *)0` rather
than a plain `0` [0]. and void pointers are required to have the same
representation and size as char pointers [1]. so this is not an issue we
need to worry about.

[0]: https://pubs.opengroup.org/onlinepubs/9699919799/basedefs/stddef.h.html#tag_13_47
[1]: https://port70.net/~nsz/c/c99/n1256.html#6.2.5p27