cmd/guru: whicherrs: don't crash on dead code

Change-Id: I5e80613ec27a4edb45516ff1ff3a88d5f9cb3a67
Reviewed-on: https://go-review.googlesource.com/21130
Reviewed-by: Alan Donovan <adonovan@google.com>

cmd/guru/testdata/src/whicherrs/main.go

@@ -21,6 +21,11 @@ func genErr(i int) error {
 	}
 }
 
+func unreachable() {
+	err := errVar // @whicherrs func-dead "err"
+	_ = err
+}
+
 func main() {
 	err := genErr(0) // @whicherrs localerrs "err"
 	_ = err

cmd/guru/testdata/src/whicherrs/main.golden

@@ -1,3 +1,6 @@
+-------- @whicherrs func-dead --------
+
+Error: pointer analysis did not find expression (dead code?)
 -------- @whicherrs localerrs --------
 this error may point to these globals:
 	errVar

cmd/guru/whicherrs.go

@@ -14,6 +14,7 @@ import (
 	"golang.org/x/tools/cmd/guru/serial"
 	"golang.org/x/tools/go/ast/astutil"
 	"golang.org/x/tools/go/loader"
+	"golang.org/x/tools/go/pointer"
 	"golang.org/x/tools/go/ssa"
 	"golang.org/x/tools/go/ssa/ssautil"
 )
@@ -141,6 +142,9 @@ func whicherrs(q *Query) error {
 
 	ptares := ptrAnalysis(ptaConfig)
 	valueptr := ptares.Queries[value]
+	if valueptr == (pointer.Pointer{}) {
+		return fmt.Errorf("pointer analysis did not find expression (dead code?)")
+	}
 	for g, v := range globals {
 		ptr, ok := ptares.Queries[v]
 		if !ok {