font/sfnt: fix argStack size check for moveto ops.

The *moveto ops don't take multiple args or arg pairs. For example, in
http://wwwimages.adobe.com/content/dam/Adobe/en/devnet/font/pdfs/5177.Type2.pdf
page 16, the rmoveto args are listed as:
dx1 dy1
which is not the same as rlineto's args, which can be repeated:
{dxa dya}+

Change-Id: I5d13f686e604955909eb0b7e52f20ce5f0522c5d
Reviewed-on: https://go-review.googlesource.com/38289
Reviewed-by: David Crawshaw <crawshaw@golang.org>

 font/sfnt/postscript.go | 20 +++++++-------------
 1 file changed, 7 insertions(+), 13 deletions(-)

diff --git a/font/sfnt/postscript.go b/font/sfnt/postscript.go
index 1a00b82..60a8294 100644
--- a/font/sfnt/postscript.go
+++ b/font/sfnt/postscript.go
@@ -875,37 +875,31 @@ func t2CAppendCubeto(p *psInterpreter, dxa, dya, dxb, dyb, dxc, dyc int32) {
 
 func t2CHmoveto(p *psInterpreter) error {
 	t2CReadWidth(p, 1)
-	if p.argStack.top < 1 {
+	if p.argStack.top != 1 {
 		return errInvalidCFFTable
 	}
-	for i := int32(0); i < p.argStack.top; i++ {
-		p.type2Charstrings.x += p.argStack.a[i]
-	}
+	p.type2Charstrings.x += p.argStack.a[0]
 	t2CAppendMoveto(p)
 	return nil
 }
 
 func t2CVmoveto(p *psInterpreter) error {
 	t2CReadWidth(p, 1)
-	if p.argStack.top < 1 {
+	if p.argStack.top != 1 {
 		return errInvalidCFFTable
 	}
-	for i := int32(0); i < p.argStack.top; i++ {
-		p.type2Charstrings.y += p.argStack.a[i]
-	}
+	p.type2Charstrings.y += p.argStack.a[0]
 	t2CAppendMoveto(p)
 	return nil
 }
 
 func t2CRmoveto(p *psInterpreter) error {
 	t2CReadWidth(p, 2)
-	if p.argStack.top < 2 || p.argStack.top%2 != 0 {
+	if p.argStack.top != 2 {
 		return errInvalidCFFTable
 	}
-	for i := int32(0); i < p.argStack.top; i += 2 {
-		p.type2Charstrings.x += p.argStack.a[i+0]
-		p.type2Charstrings.y += p.argStack.a[i+1]
-	}
+	p.type2Charstrings.x += p.argStack.a[0]
+	p.type2Charstrings.y += p.argStack.a[1]
 	t2CAppendMoveto(p)
 	return nil
 }