Validate socket FD & optimize recv context.

Validate socket FD & optimize recv context.

From the recv ctx callback, the socket can be closed.
But the glib cannot detect the closed FD correctly.
(It doesn't toggles G_IO_HUP, G_IO_ERR, G_IO_NVAL bits even though I closed the FD)
So I checked the FD from the callback explicitly, before return from it.
However this patch will be available only for the Single-Thread loop. (com-core)

And the receive context is optimized.
Every single loop after done to prepare A receive context,
it will be destroyed. But this patch will reuse it instead of destroying it.
So there is no need to reallocate same memory again.
It will save few milli (or micro) seconds.

for the future-work.
I'll optimize the "packet" creating code.
It can be reused too after finish the processing of receive context.
Then we can save more few milli(micro) seconds (for reallocating time).

Each execution takes small time.
But this scenario occupies almost time for communicating with other processes.
it means, even though it can save few milli(micro) secs for each time.
The total saved time will be meaningful.

Change-Id: Ibb3adc715090b6d78a5cfeb48d28463f35b9a0f4