[release-branch.go1.4] runtime: don't fail if we find a pointer to an invalid span on 32 bit
The 32-bit heap may have holes in it. Pointers to (non-heap) objects
in those holes shouldn't cause the GC to throw.
This change is somewhat of a band-aid fix for 1.4.2. We should do
a more thorough fix for tip (keep track of the holes in the heap
with special MSpans, say).