go/analysis: avoid panic in ifaceassert

Presently ifaceassert.go panics if given asserts whose
types reduce to
[]interface{}.(string)
which is illegal Go. Its tests won't even run on such code, but
gopls will happily invoke it. This CL adds a test for nil.

See golang/go#42857

Change-Id: I2791f4bd0b58559e65e6590822ac8f4123989273
Reviewed-on: https://go-review.googlesource.com/c/tools/+/273766
Reviewed-by: Michael Matloob <matloob@golang.org>
Trust: Peter Weinberger <pjw@google.com>

go/analysis/passes/ifaceassert/ifaceassert.go

@@ -41,6 +41,10 @@ var Analyzer = &analysis.Analyzer{
 // assertableTo checks whether interface v can be asserted into t. It returns
 // nil on success, or the first conflicting method on failure.
 func assertableTo(v, t types.Type) *types.Func {
+	if t == nil || v == nil {
+		// not assertable to, but there is no missing method
+		return nil
+	}
 	// ensure that v and t are interfaces
 	V, _ := v.Underlying().(*types.Interface)
 	T, _ := t.Underlying().(*types.Interface)