Added mutex protection and null check for removeFromGroup

[ethouris]

Fixes #1608

The removeFromGroup could be called in the meantime or somewhere from inside connectIn call, possibly also in the receiver worker thread as a reaction on connection timeout. The call that has caused a crash was in the exception handler, which stated that the m_IncludedGroup field is still valid as set before. To prevent things like this from happening, this function now rewrites the m_IncludedGroup and reads it under protection. The following call to CUDTGroup::remove is also protected by a group mutex and can be called multiple times (the call on an already removed socket is ignored).

[maxsharabayko]

Reduced nesting:

void CUDTSocket::removeFromGroup(bool broken)
{
    // First lock access and clear pointers
    enterCS(m_ControlLock);
    CUDTGroup* pg = m_IncludedGroup;
    m_IncludedIter = CUDTGroup::gli_NULL();
    m_IncludedGroup = NULL;
    leaveCS(m_ControlLock);

    if (!pg)
        return;

    pg->remove(m_SocketID);
    if (broken)
    {
        // Activate the SRT_EPOLL_UPDATE event on the group
        // if it was because of a socket that was earlier connected
        // and became broken. This is not to be sent in case when
        // it is a failure during connection, or the socket was
        // explicitly removed from the group.
        pg->activateUpdateEvent();
    }
}

[ethouris]

This reduces clarity as well. A critical section inside braces marks it clearer than the pair of enter/leave calls. The return on no pg looks good.

[maxsharabayko]

Wherever there is a code block without ifs, whiles, etc., it means there should have been a function.

// A sample name, not pretending to be good
CUDTGroup* CUDTSocket::clearGroupMembership()
{
    ScopedLock grd (m_ControlLock);
    CUDTGroup* pg = m_IncludedGroup;
    m_IncludedIter = CUDTGroup::gli_NULL();
    m_IncludedGroup = NULL;
    return pg;
}

void CUDTSocket::removeFromGroup(bool broken)
{
    // First lock access and clear pointers
    CUDTGroup* pg = clearGroupMembership();
    if (!pg)
        return;

    pg->remove(m_SocketID);
    if (broken)
    {
        // Activate the SRT_EPOLL_UPDATE event on the group
        // if it was because of a socket that was earlier connected
        // and became broken. This is not to be sent in case when
        // it is a failure during connection, or the socket was
        // explicitly removed from the group.
        pg->activateUpdateEvent();
    }
}

[maxsharabayko]

No more comments except for the suggestion above.