go/types: handle nil pointer when panic is written outside of a function

The current implementation crashes when someone writes a panic outside of
a function, which makes sense since that is broken code. This fix allows
one to type-check broken code.

Updates #22467

Change-Id: I81b90dbd918162a20c60a821340898eaf02e648d
Reviewed-on: https://go-review.googlesource.com/132235


Reviewed-by: Alan Donovan <adonovan@google.com>