fmt: set p.field before nil check

Fixes #3752.

R=golang-dev, rsc
CC=golang-dev
https://golang.org/cl/6331062

src/pkg/fmt/fmt_test.go

@@ -851,3 +851,15 @@ func TestIsSpace(t *testing.T) {
 		}
 	}
 }
+
+func TestNilDoesNotBecomeTyped(t *testing.T) {
+	type A struct{}
+	type B struct{}
+	var a *A = nil
+	var b B = B{}
+	got := Sprintf("%s %s %s %s %s", nil, a, nil, b, nil)
+	const expect = "%!s(<nil>) %!s(*fmt_test.A=<nil>) %!s(<nil>) {} %!s(<nil>)"
+	if got != expect {
+		t.Errorf("expected:\n\t%q\ngot:\n\t%q", expect, got)
+	}
+}

src/pkg/fmt/print.go

@@ -712,6 +712,9 @@ func (p *pp) handleMethods(verb rune, plus, goSyntax bool, depth int) (wasString
 }
 
 func (p *pp) printField(field interface{}, verb rune, plus, goSyntax bool, depth int) (wasString bool) {
+	p.field = field
+	p.value = reflect.Value{}
+
 	if field == nil {
 		if verb == 'T' || verb == 'v' {
 			p.buf.Write(nilAngleBytes)
@@ -721,8 +724,6 @@ func (p *pp) printField(field interface{}, verb rune, plus, goSyntax bool, depth
 		return false
 	}
 
-	p.field = field
-	p.value = reflect.Value{}
 	// Special processing considerations.
 	// %T (the value's type) and %p (its address) are special; we always do them first.
 	switch verb {