fmt: fix crash for Printf("%.", 3)

Fixes #5311

R=golang-dev, bradfitz, iant
CC=golang-dev
https://golang.org/cl/8961050

src/pkg/fmt/fmt_test.go

@@ -497,6 +497,9 @@ var fmttests = []struct {
 	// causing +2+0i and +3+0i instead of 2+0i and 3+0i.
 	{"%v", []complex64{1, 2, 3}, "[(1+0i) (2+0i) (3+0i)]"},
 	{"%v", []complex128{1, 2, 3}, "[(1+0i) (2+0i) (3+0i)]"},
+
+	// Incomplete format specification caused crash.
+	{"%.", 3, "%!.(int=3)"},
 }
 
 func TestSprintf(t *testing.T) {

src/pkg/fmt/print.go

@@ -1072,7 +1072,7 @@ func (p *pp) doPrintf(format string, a []interface{}) {
 			p.fmt.wid, p.fmt.widPresent, i = parsenum(format, i, end)
 		}
 		// do we have precision?
-		if i < end && format[i] == '.' {
+		if i+1 < end && format[i] == '.' {
 			if format[i+1] == '*' {
 				p.fmt.prec, p.fmt.precPresent, i, fieldnum = intFromArg(a, end, i+1, fieldnum)
 				if !p.fmt.precPresent {