Save r3 before using it for the trap check, else we end up saving the new r3,
containing the trap instruction encoding (0x7c810808), and restoring it back
with the frame on return.  This caused it to panic on my ppc32 machine, but
somehow my ppc64 machine overlooked it, because I was using such a simple
dtrace probe.

X-MFC-with:	r259245
MFC after:	2 weeks
Revert r259045.

It seems that clang miscompiles code related to UDP with -fno-strict-overflow.