[psaux] Fix numeric overflow.

Reported as

  https://bugs.chromium.org/p/oss-fuzz/issues/detail?id=10768

* src/psaux/cffdecode.c (cff_decoder_parse_charstrings) <cff_op_roll>
[CFF_CONFIG_OPTION_OLD_ENGINE]: Use NEG_INT.

ChangeLog

@@ -1,3 +1,14 @@
+2018-10-02  Werner Lemberg  <wl@gnu.org>
+
+	[psaux] Fix numeric overflow.
+
+	Reported as
+
+	  https://bugs.chromium.org/p/oss-fuzz/issues/detail?id=10768
+
+	* src/psaux/cffdecode.c (cff_decoder_parse_charstrings) <cff_op_roll>
+	[CFF_CONFIG_OPTION_OLD_ENGINE]: Use NEG_INT.
+
 2018-10-02  Werner Lemberg  <wl@gnu.org>
 
 	[pshinter] Handle numeric overflow.

src/psaux/cffdecode.c

@@ -1839,7 +1839,7 @@
               /* before C99 it is implementation-defined whether    */
               /* the result of `%' is negative if the first operand */
               /* is negative                                        */
-              idx = -( ( -idx ) % count );
+              idx = -( NEG_INT( idx ) % count );
               while ( idx < 0 )
               {
                 FT_Fixed  tmp = args[0];