cmd/gc: fix crash during escape analysis

cmd/gc: fix crash during escape analysis

Fixes #10441

Compiler crashed as:

panic: runtime error: invalid memory address or nil pointer dereference
goroutine 1 [running]:
cmd/internal/gc.haspointers(0x0, 0xffffffffffffffff)
    src/cmd/internal/gc/reflect.go:623 +0x1f
cmd/internal/gc.escwalk(0x20c82dc00, 0xffffffffffffffff, 0x20c82dda0, 0x20c835520)
    src/cmd/internal/gc/esc.go:1291 +0x14aa
cmd/internal/gc.escwalk(0x20c82dc00, 0xffffffffffffffff, 0x20c82dda0, 0x20beac340)
    src/cmd/internal/gc/esc.go:1386 +0x836

while evaluating haspointers of the fake OADDR node here:

	leaks = level <= 0 && dst.Escloopdepth < src.Escloopdepth || level < 0 && dst == &e.funcParam && haspointers(src.Type)

Change-Id: I13c23fced52e8fcd0ae1df81b374df8632eb7790
Reviewed-on: https://go-review.googlesource.com/8932
Reviewed-by: David Chase <drchase@google.com>
Run-TryBot: Dmitry Vyukov <dvyukov@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>