[cff] Fix heap buffer overflow in old engine.

Reported as

  https://bugs.chromium.org/p/oss-fuzz/issues/detail?id=9967

* src/psaux/cffdecode.c (cff_decoder_parse_charstrings)
<cff_op_blend> [CFF_CONFIG_OPTION_OLD_ENGINE]: `num_designs' must be
non-zero.

ChangeLog

@@ -1,3 +1,15 @@
+2018-08-14  Werner Lemberg  <wl@gnu.org>
+
+	[cff] Fix heap buffer overflow in old engine.
+
+	Reported as
+
+	  https://bugs.chromium.org/p/oss-fuzz/issues/detail?id=9967
+
+	* src/psaux/cffdecode.c (cff_decoder_parse_charstrings)
+	<cff_op_blend> [CFF_CONFIG_OPTION_OLD_ENGINE]: `num_designs' must be
+	non-zero.
+
 2018-08-16  Young Xiao  <yangx92@hotmail.com>
 
 	* builds/mac/ftmac.c (parse_fond): Fix buffer overrun.

src/psaux/cffdecode.c

@@ -1935,6 +1935,7 @@
         case cff_op_blend:
           /* this operator was removed from the Type2 specification */
           /* in version 16-March-2000                               */
+          if ( num_designs )
           {
             FT_Int  num_results = (FT_Int)( args[0] >> 16 );
 
@@ -1953,6 +1954,8 @@
             args     -= num_results * ( num_designs - 1 );
             num_args -= num_results * ( num_designs - 1 );
           }
+          else
+            goto Syntax_Error;
           break;
 
         case cff_op_dotsection: