Don't crash in Sym.ReceiverName for symbols like "x.x"

R=rsc
APPROVED=rsc
DELTA=2  (0 added, 0 deleted, 2 changed)
OCL=34404
CL=34406

src/pkg/debug/gosym/symtab.go

@@ -52,7 +52,7 @@ func (s *Sym) PackageName() string {
 func (s *Sym) ReceiverName() string {
 	l := strings.Index(s.Name, ".");
 	r := strings.LastIndex(s.Name, ".");
-	if l == -1 || r == -1 {
+	if l == -1 || r == -1 || l == r {
 		return "";
 	}
 	return s.Name[l+1:r];

usr/austin/sym/gosymtab.go

@@ -63,7 +63,7 @@ func (c *CommonSym) PackageName() string {
 func (c *CommonSym) ReceiverName() string {
 	l := strings.Index(c.Name, "·");
 	r := strings.LastIndex(c.Name, "·");
-	if l == -1 || r == -1 {
+	if l == -1 || r == -1 || l == r {
 		return "";
 	}
 	return c.Name[l+len("·"):r];