Initialize m_parent in SocketRequestHandler ctor

Static analysis detected that the member variable
was not initialized in the ctor.  While a cursory
review of the derived classes show that it would
be initialized when appropriate, initialize it to
NULL in the ctor to make static analysis happy.

Fixes Coverity 700891

Fixes 11676.

Signed-off-by: Paul Harrison <pharrison@mythtv.org>