We canonicalize repeats that cover the whole range: "0:0:0/1" → "0:0:*". But
we'd also do "0:0:0/1,0" → "0:0:*,0", which we then refuse to parse. Thus,
first go throug the whole chain, and print a '*' and nothing else if any of the
components covers the whole range.
0..3 is not the same as 0..infinity, we need to check both ends of the range.
This logic was added in 3215e35c40, and back then
the field was called .value. .stop was added later and apparently wasn't taken
into account here.
Coverage data shows that we didn't test calendar_spec_next_usec() and
associated functions at all.
The input samples so far were only used until the first NUL. We take advantage
of that by using the part until the second NUL as the starting timestamp,
retaining backwards compatibility for how the first part is used.
Fixes: oss-fuzz#22208
```
test/fuzz/fuzz-calendarspec/oss-fuzz-22208... ../src/shared/calendarspec.c:666:48: runtime error: signed integer overflow: 2147000000 + 1000000 cannot be represented in type 'int'
#0 0x7f0b9f6cc56a in prepend_component ../src/shared/calendarspec.c:666
#1 0x7f0b9f6cd03a in parse_chain ../src/shared/calendarspec.c:718
#2 0x7f0b9f6cea1c in parse_calendar_time ../src/shared/calendarspec.c:845
#3 0x7f0b9f6d1397 in calendar_spec_from_string ../src/shared/calendarspec.c:1084
#4 0x401570 in LLVMFuzzerTestOneInput ../src/fuzz/fuzz-calendarspec.c:17
#5 0x401ae0 in main ../src/fuzz/fuzz-main.c:39
#6 0x7f0b9e31b1a2 in __libc_start_main (/lib64/libc.so.6+0x271a2)
#7 0x40122d in _start (/home/fsumsal/repos/systemd/build/fuzz-calendarspec+0x40122d)
SUMMARY: UndefinedBehaviorSanitizer: undefined-behavior ../src/shared/calendarspec.c:666:48 in
```
