refactor/satisfy: check for presence of types.Info maps

Although Finder.Find's doc says that it requires certain maps in
its argument types.Info, it didn't actually check. Now it does.

Change-Id: I4938f9cbca9377b935d8a027d59438b9697558b7
Reviewed-on: https://go-review.googlesource.com/c/tools/+/662698
Reviewed-by: Alan Donovan <[email protected]>
LUCI-TryBot-Result: Go LUCI <[email protected]>

Author: jba

refactor/satisfy/find.go

@@ -84,6 +84,9 @@ type Finder struct {
 // info.{Defs,Uses,Selections,Types} must have been populated by the
 // type-checker.
 func (f *Finder) Find(info *types.Info, files []*ast.File) {
+	if info.Defs == nil || info.Uses == nil || info.Selections == nil || info.Types == nil {
+		panic("Finder.Find: one of info.{Defs,Uses,Selections.Types} is not populated")
+	}
 	if f.Result == nil {
 		f.Result = make(map[Constraint]bool)
 	}