Fix handling of invalid format 2 cmaps.

The problem was introduced after the last release.

Reported as

  https://bugs.chromium.org/p/oss-fuzz/issues/detail?id=7828

* src/sfnt/ttcmap.c (tt_cmap2_char_next): Avoid endless loop.

ChangeLog

@@ -1,3 +1,15 @@
+2018-03-13  Werner Lemberg  <wl@gnu.org>
+
+	Fix handling of invalid format 2 cmaps.
+
+	The problem was introduced after the last release.
+
+	Reported as
+
+	  https://bugs.chromium.org/p/oss-fuzz/issues/detail?id=7828
+
+	* src/sfnt/ttcmap.c (tt_cmap2_char_next): Avoid endless loop.
+
 2018-04-17  Werner Lemberg  <wl@gnu.org>
 
 	[truetype] Integer overflow issues.

src/sfnt/ttcmap.c

@@ -518,6 +518,13 @@
         FT_UInt   pos, idx;
 
 
+        if ( char_lo > start + count )
+        {
+          /* this happens only for a malformed cmap */
+          charcode = 0x100;
+          continue;
+        }
+
         if ( offset == 0 )
         {
           if ( charcode == 0x100 )