cmd/vet: warn about zero arg index in print format

Argument indexes in a format string are one-based, however vet would not
warn when using a zero-index unless the type of the argument referenced
was not a string. That warning was misleading as it would say the type
was not a string. Vet will now print a correct warning when using a zero
index.

Included are tests for both cases.

Fixes #9752

Change-Id: I285e99990a86a653b4668b0c279d5f5f1c34f7aa
Reviewed-on: https://go-review.googlesource.com/3692
Reviewed-by: Rob Pike <r@golang.org>

cmd/vet/print.go

@@ -474,6 +474,10 @@ func (f *File) argCanBeChecked(call *ast.CallExpr, formatArg int, isStar bool, s
 		// Shouldn't happen, so catch it with prejudice.
 		panic("negative arg num")
 	}
+	if argNum == 0 {
+		f.Badf(call.Pos(), `index value [0] for %s("%s"); indexes start at 1`, state.name, state.format)
+		return false
+	}
 	if argNum < len(call.Args)-1 {
 		return true // Always OK.
 	}

cmd/vet/testdata/print.go

@@ -174,6 +174,8 @@ func PrintfTests() {
 	Printf("%[3]*s", "hi", 2)            // ERROR "missing argument for Printf.* reads arg 3, have only 2"
 	fmt.Sprintf("%[3]d", 2)              // ERROR "missing argument for Sprintf.* reads arg 3, have only 1"
 	Printf("%[2]*.[1]*[3]d", 2, "hi", 4) // ERROR "arg .hi. for \* in printf format not of type int"
+	Printf("%[0]s", "arg1")              // ERROR "index value \[0\] for Printf.*; indexes start at 1"
+	Printf("%[0]d", 1)                   // ERROR "index value \[0\] for Printf.*; indexes start at 1"
 	// Something that satisfies the error interface.
 	var e error
 	fmt.Println(e.Error()) // ok