godoc: fix crash in -analysis

Fixes golang/go#20641

Change-Id: I499b4b8da2899c4b606ce3eedd30caab57607a85
Reviewed-on: https://go-review.googlesource.com/45570
Reviewed-by: Robert Griesemer <gri@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

Author: adonovan

godoc/analysis/analysis.go

@@ -299,7 +299,7 @@ type analysis struct {
 	result    *Result
 	prog      *ssa.Program
 	ops       []chanOp       // all channel ops in program
-	allNamed  []*types.Named // all named types in the program
+	allNamed  []*types.Named // all "defined" (formerly "named") types in the program
 	ptaConfig pointer.Config
 	path2url  map[string]string // maps openable path to godoc file URL (/src/fmt/print.go)
 	pcgs      map[*ssa.Package]*packageCallGraph
@@ -475,7 +475,9 @@ func Run(pta bool, result *Result) {
 	for _, info := range iprog.AllPackages {
 		for _, obj := range info.Defs {
 			if obj, ok := obj.(*types.TypeName); ok {
-				a.allNamed = append(a.allNamed, obj.Type().(*types.Named))
+				if named, ok := obj.Type().(*types.Named); ok {
+					a.allNamed = append(a.allNamed, named)
+				}
 			}
 		}
 	}