journal-upload: add asserts that snprintf does not return an error

journal-upload: add asserts that snprintf does not return an error

LGMT complains:
> The size argument of this snprintf call is derived from its return value,
> which may exceed the size of the buffer and overflow.

Let's make sure that r is non-negative. (This shouldn't occur unless the format
string is borked, so let's just add an assert.)
Then, let's reorder the comparison to avoid the potential overflow.