Avoids buffer overflow in xmlrpcpp if fd > 1024

[kmactavish]

A temporary fix to avoid buffer overflow in XmlRpcDispatch, since select() only works for file descriptors < FD_SETSIZE (1024). If fd >= FD_SETSIZE, there is buffer overflow, memory corruption, and all-round undefined behaviour. The real solution is to switch to poll as in #833, but since that PR was reverted in #981, this avoids catastrophic failure until that can be worked out.

It throws an exception since:

• this behaviour is no less intrusive than a buffer overflow
• returning an XmlRpcError was handled silently, and just looked like ros comm not being connected
• this is a temporary solution to fail in a more predictable way.

[kmactavish]

The build of 582984f failed due to a test added by #1014. Even with this entire change commented out.

08:55:05 [test_roscpp.rosunit-timer_callbacks/multipleSteadyTimeCallbacks][FAILURE]------
08:55:05 /tmp/catkin_workspace/src/ros_comm/test/test_roscpp/test/src/timer_callbacks.cpp:220
08:55:05 Failed

[kmactavish]

This was superseded by #1133 .