[psaux] Fix timeout in old CFF engine.

Reported as

  https://bugs.chromium.org/p/oss-fuzz/issues/detail?id=11260

* src/psaux/cffdecode.c (cff_decoder_parse_charstrings)
<cff_op_sqrt> [CFF_CONFIG_OPTION_OLD_ENGINE]: Fix potential endless
loop.

ChangeLog

@@ -1,3 +1,15 @@
+2018-11-06  Werner Lemberg  <wl@gnu.org>
+
+	[psaux] Fix timeout in old CFF engine.
+
+	Reported as
+
+	  https://bugs.chromium.org/p/oss-fuzz/issues/detail?id=11260
+
+	* src/psaux/cffdecode.c (cff_decoder_parse_charstrings)
+	<cff_op_sqrt> [CFF_CONFIG_OPTION_OLD_ENGINE]: Fix potential endless
+	loop.
+
 2018-11-04  Alexei Podtelezhnikov  <apodtele@gmail.com>
 
 	* src/truetype/ttgxvar.c: Use enum definitions.

src/psaux/cffdecode.c

@@ -1748,7 +1748,10 @@
         case cff_op_sqrt:
           FT_TRACE4(( " sqrt\n" ));
 
-          if ( args[0] > 0 )
+          /* without upper limit the loop below might not finish */
+          if ( args[0] > 0x7FFFFFFFL )
+            args[0] = 46341;
+          else if ( args[0] > 0 )
           {
             FT_Fixed  root = args[0];
             FT_Fixed  new_root;