datetime: fix precision overflow

We didn't take into consideration the fact, that precision
value passed to control the width of nanoseconds part in
datetime_object:format could be more than maximum positive
value, integer may have. Currently it leads to segfault.

```
tarantool> require('datetime').new{}:format('%2147483648f')
```

We should check errno in order to find out, if overflow
occurs. The problem is the fact, that `width` variable must
have int type due to snprintf requirements ("%*d") and
strtol returns long. Errno won't be set if returned value
is in bounds [INT_MAX, LONG_MAX], but it will overflow
int resulting in inconsistent behavior.

So, let's save the result of strotl to the temp value.
If this value doesn't belong to the above-mentioned set,
or errno was set, we assign to `width` maximum value, it
may have: 9.

Closes tarantool/security#31

NO_DOC=bugfix

(cherry picked from commit b6159217)