pkg/runtime: 2 sanity checks in the runtime-gdb.py prettyprinters.
pkg/runtime: 2 sanity checks in the runtime-gdb.py prettyprinters.

Don't try to print obviously corrupt slices or interfaces.
Doesn't actually solve 3047 or 2818, but seems a good idea anyway.

R=rsc, bsiegert
CC=golang-dev
http://codereview.appspot.com/5708061

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