datetime: fix strptime for fuzzer

Google fuzzing efforts revealed yet another bound condition
we don't handle well in the `tnt_strptime` function:

- for format `%m%g%W`;
- and input string `07001`.

We failed with assertion failure:
```
  | datetime_strptime_fuzzer: ./src/lib/core/datetime.c:148: \
_Bool tm_to_datetime(struct tnt_tm *, struct datetime *): \
Assertion `mday >= 1 && mday <= 31' failed.
```

Closes #8525

NO_TEST=updated fuzzer corpus
NO_CHANGELOG=internal
NO_DOC=internal

changelogs/unreleased/gh-8525-strptime-fix.md

+## bugfix/datetime
+
+* Fixed a bug in `strptime` when the assertion was triggered (gh-8525).

src/lib/tzcode/strptime.c

@@ -627,12 +627,11 @@ tnt_strptime(const char *__restrict buf, const char *__restrict fmt,
 	}
 
 	if (!(flags & FLAG_YDAY) && (flags & FLAG_YEAR)) {
-		if ((flags & (FLAG_MONTH | FLAG_MDAY)) ==
-		    (FLAG_MONTH | FLAG_MDAY)) {
+		if ((flags & FLAG_MONTH) != 0 ) {
 			tm->tm_yday = start_of_month[isleap(tm->tm_year +
 							    TM_YEAR_BASE)]
 						    [tm->tm_mon] +
-				(tm->tm_mday - 1);
+				(tm->tm_mday - 1) * !!(flags & FLAG_MDAY);
 			flags |= FLAG_YDAY;
 		} else if (day_offset != -1) {
 			int tmpwday, tmpyday, fwo;

test/static/corpus/datetime_strptime/clusterfuzz-6575551489114112

+07001\ %m%g%W
\ No newline at end of file