runtime: avoid reference to stale stack after GC shrinkstack

runtime: avoid reference to stale stack after GC shrinkstack

Dangling pointer error. Unlikely to trigger in practice, but still.
Found by running GODEBUG=efence=1 GOGC=1 trace.test.

Change-Id: Ice474dedcf62dd33ab77526287a023ba3b166db9
Reviewed-on: https://go-review.googlesource.com/12991
Reviewed-by: Austin Clements <austin@google.com>