runtime: fix out-of-bounds in stack debugging

Currently stackDebug=4 crashes as:

panic: runtime error: index out of range
fatal error: panic on system stack
runtime stack:
runtime.throw(0x607470, 0x15)
	src/runtime/panic.go:527 +0x96
runtime.gopanic(0x5ada00, 0xc82000a1d0)
	src/runtime/panic.go:354 +0xb9
runtime.panicindex()
	src/runtime/panic.go:12 +0x49
runtime.adjustpointers(0xc820065ac8, 0x7ffe58b56100, 0x7ffe58b56318, 0x0)
	src/runtime/stack1.go:428 +0x5fb
runtime.adjustframe(0x7ffe58b56200, 0x7ffe58b56318, 0x1)
	src/runtime/stack1.go:542 +0x780
runtime.gentraceback(0x487760, 0xc820065ac0, 0x0, 0xc820001080, 0x0, 0x0, 0x7fffffff, 0x6341b8, 0x7ffe58b56318, 0x0, ...)
	src/runtime/traceback.go:336 +0xa7e
runtime.copystack(0xc820001080, 0x1000)
	src/runtime/stack1.go:616 +0x3b1
runtime.newstack()
	src/runtime/stack1.go:801 +0xdde

Change-Id: If2d60960231480a9dbe545d87385fe650d6db808
Reviewed-on: https://go-review.googlesource.com/12763


Reviewed-by: Russ Cox <rsc@golang.org>