Temporal to_string() returns a String with unset length when allocation fails

The to_string() methods of Temporal_hybrid, Interval_DDhhmmssff, Time, Date and Datetime in sql/sql_type.h all have the same body:

str->set_charset(&my_charset_numeric);

if (!str->alloc(MAX_DATE_STRING_REP_LENGTH))

  str->length(my_TIME_to_str(this, const_cast<char*>(str->ptr()), dec));

return str;

When String::alloc() fails (out of memory) the length is not set, but str is still returned as success. The caller then uses a String whose buffer may be NULL and whose length is whatever it was before.

Expected: return NULL on allocation failure, like the !is_valid_temporal() case.

The body is copied five times and only the formatter differs, so it is worth factoring into one helper so that a fix cannot miss a copy.

Present in 10.11 and later.

Found during review of MDEV-25529 (https://github.com/MariaDB/server/pull/5294); the code predates that change.