[psaux] Better protect `flex' handling.

Reported as

  https://bugs.chromium.org/p/oss-fuzz/issues/detail?id=935

* src/psaux/t1decode.c (t1_decoder_parse_charstrings)
<callothersubr>: Since there is not a single flex operator but a
series of subroutine calls, malformed fonts can call arbitrary other
operators after the start of a flex, possibly adding points.  For
this reason we have to check the available number of points before
inserting a point.

ChangeLog

@@ -1,3 +1,18 @@
+2017-03-23  Werner Lemberg  <wl@gnu.org>
+
+	[psaux] Better protect `flex' handling.
+
+	Reported as
+
+	  https://bugs.chromium.org/p/oss-fuzz/issues/detail?id=935
+
+	* src/psaux/t1decode.c (t1_decoder_parse_charstrings)
+	<callothersubr>: Since there is not a single flex operator but a
+	series of subroutine calls, malformed fonts can call arbitrary other
+	operators after the start of a flex, possibly adding points.  For
+	this reason we have to check the available number of points before
+	inserting a point.
+
 2017-03-23  Werner Lemberg  <wl@gnu.org>
 
 	[sfnt] Fix check for default named instance.

src/psaux/t1decode.c

@@ -780,10 +780,19 @@
             /* point without adding any point to the outline    */
             idx = decoder->num_flex_vectors++;
             if ( idx > 0 && idx < 7 )
+            {
+              /* in malformed fonts it is possible to have other */
+              /* opcodes in the middle of a flex (which don't    */
+              /* increase `num_flex_vectors'); we thus have to   */
+              /* check whether we can add a point                */
+              if ( FT_SET_ERROR( t1_builder_check_points( builder, 1 ) ) )
+                goto Syntax_Error;
+
               t1_builder_add_point( builder,
                                     x,
                                     y,
                                     (FT_Byte)( idx == 3 || idx == 6 ) );
+            }
           }
           break;