font/sfnt: fix proprietary fonts and cmap format 12.

Two recent commits ("proprietary fonts" and "cmap format 12") each
passed all of its own tests, but the combination wasn't tested until
both were submitted.

Change-Id: Ic4c2ae8deb1e4623ca5543672dc46d55bfce91a4
Reviewed-on: https://go-review.googlesource.com/36372
Reviewed-by: David Crawshaw <crawshaw@golang.org>

font/sfnt/cmap.go

@@ -206,13 +206,13 @@ func (f *Font) makeCachedGlyphIndexFormat12(buf []byte, offset, _ uint32) ([]byt
 	if err != nil {
 		return nil, err
 	}
-	offset += headerSize
-
 	length := u32(buf[4:])
-	numGroups := u32(buf[12:])
 	if f.cmap.length < offset || length > f.cmap.length-offset {
 		return nil, errInvalidCmapTable
 	}
+	offset += headerSize
+
+	numGroups := u32(buf[12:])
 	if numGroups > maxCmapSegments {
 		return nil, errUnsupportedNumberOfCmapSegments
 	}

font/sfnt/sfnt.go

@@ -30,8 +30,17 @@ import (
 const (
 	// This value is arbitrary, but defends against parsing malicious font
 	// files causing excessive memory allocations. For reference, Adobe's
-	// SourceHanSansSC-Regular.otf has 65535 glyphs and 1581 cmap segments.
+	// SourceHanSansSC-Regular.otf has 65535 glyphs and:
-	maxCmapSegments = 4096
+	//	- its format-4  cmap table has  1581 segments.
+	//	- its format-12 cmap table has 16498 segments.
+	//
+	// TODO: eliminate this constraint? If the cmap table is very large, load
+	// some or all of it lazily (at the time Font.GlyphIndex is called) instead
+	// of all of it eagerly (at the time Font.initialize is called), while
+	// keeping an upper bound on the memory used? This will make the code in
+	// cmap.go more complicated, considering that all of the Font methods are
+	// safe to call concurrently, as long as each call has a different *Buffer.
+	maxCmapSegments = 20000
 
 	maxGlyphDataLength  = 64 * 1024
 	maxHintBits         = 256