RT 75082: recv() with MSG_TRUNC flag SEGV

RT 75082: recv() with MSG_TRUNC flag SEGV

The recv() system call, with the MSG_TRUNC flag, returns the
true size of the packet, even if it is larger than the supplied buffer.
Since perl's equivalent recv() function doesn't return the size
(apart from what's implied as the returned length of the buffer), there
doesn't seem to to be any way to return this value to the caller. So
silently ignore it.

Before, we were setting SvCUR to the size, even if it was larger than
the buffer. Which was Bad.