Database errors can go unnoticed with DBselect(), DBfetch() functions

1. DBselect() blocks if database is down, but may return NULL in case of other errors.
In many functions there are correct checks like "if (NULL == (result = DBselect(....)", that is good.
In some functions DBselect() result is passed directly to DBfetch() in a code like this:
"if (NULL == (row = DBfetch(result))" or "while (NULL != (row = DBfetch(result)))".
In these cases a database error is masked as "no (more) records found", which may cause errors in program logic. This was the root cause of ZBX-12046. Another example is get_value_internal(), which will return 0 as a value instead of NOTSUPPORTED in case of DB error.
2. sasha pointed out a similar problem with DBfetch().
In case of Oracle DB the DBfetch() retrieves data in chunks from server. If a database error occurs, DBfetch() returns NULL, interpreted as "no (more) records found", which may not always be true.