cmd/compile: fix crash on write barrier insertion

The compiler can crash if the compiled code tries to
unconditionally read from a nil pointer. This should cause
the generated binary to panic, not the compiler.

Fixes #33438

Change-Id: Ic8fa89646d6968e2cc4e27da0ad9286662f8bc49
Reviewed-on: https://go-review.googlesource.com/c/go/+/188760


Reviewed-by: Austin Clements <austin@google.com>