cmd/pprof: fix symbol resolution for remote profiles

Fixes #9199.

LGTM=iant
R=golang-codereviews, iant
CC=austin, golang-codereviews, minux
https://golang.org/cl/183080043

src/cmd/pprof/internal/symbolizer/symbolizer.go

@@ -32,6 +32,10 @@ func Symbolize(mode string, prof *profile.Profile, obj plugin.ObjTool, ui plugin
 		}
 	}
 
+	if len(prof.Mapping) == 0 {
+		return fmt.Errorf("no known mappings")
+	}
+
 	mt, err := newMapping(prof, obj, ui, force)
 	if err != nil {
 		return err