Fix bug with type inference when shortcut return type syntax is used.

Fixes #469.

_testing/DESC

@@ -57,3 +57,4 @@ test.0056 - type alias method propagation
 test.0057 - embedded interfaces
 test.0058 - canonical import aliases
 test.0059 - canonical import aliases, more complicated case (doesn't work as you might expect)
+test.0060 - shortcut syntax for return value types in functions

_testing/test.0060/out.expected

@@ -0,0 +1,2 @@
+Found 1 candidates:
+  func Read(p []byte) (n int, err error)

_testing/test.0060/test.go.in

@@ -0,0 +1,10 @@
+package main
+
+import "io"
+
+func f() (a, b io.Reader, c error) { return a }
+
+func main() {
+	a, b, c := f()
+	b.
+}

decl.go

@@ -514,17 +514,14 @@ func func_return_type(f *ast.FuncType, index int) ast.Expr {
 	i := 0
 	var field *ast.Field
 	for _, field = range f.Results.List {
-		if i >= index {
-			return field.Type
-		}
+		n := 1
 		if field.Names != nil {
-			i += len(field.Names)
-		} else {
-			i++
+			n = len(field.Names)
 		}
-	}
-	if i >= index {
-		return field.Type
+		if i <= index && index < i+n {
+			return field.Type
+		}
+		i += n
 	}
 	return nil
 }