zzip: string: fix invalid malloc attribute

zzip: string: fix invalid malloc attribute

The `_zzip_strnlen` function is annotated with the `zzip_new__` macro,
which is defined to `__attribute__((malloc))` on systems which support
this attribute. The malloc attribute can be applied to functions that
return a pointer value, indicating that a returned non-NULL pointer
cannot alias and that its contents have undefined content. But as
`_zzip_strnlen` neither performs an allocation nor returns a pointer to
memory, we cannot use that attribute here.

Remove it to fix a compiler warning.