Prevent API resolving parent with unresolved children

There are discrepancies between issue updates from the GUI vs the API,
as bug_udpate.php has not been kept in sync with mc_issue_update().

This is a quick fix for one of these occurrences; REST/SOAP API allowed
mc_issue_update() to resolve a parent Issue having unresolved children
while the GUI does not (unless $g_allow_parent_of_unresolved_to_close
is ON).

The proper, long-term solution to this problem is to implement a new
IssueUpdateCommand to ensure consistent behavior (see Issue 0032861).

Fixes 0032860