[truetype] Fix handling of packed deltas in Variation Fonts.

* src/truetype/ttgxvar (ft_var_readpackeddeltas): Don't expect the number of
bytes used to encode the deltas to be higher than the number of encoded
values.  The specification allows a very compact encoding; for example, a
list of 200 zeros can be encoded with just a couple of bytes.

We now count the consumed bytes to make sure to not read more than expected.

src/truetype/ttgxvar.c

@@ -264,55 +264,80 @@
     FT_Fixed  *deltas = NULL;
     FT_UInt    runcnt, cnt;
     FT_UInt    i, j;
+    FT_UInt    bytes_used;
     FT_Memory  memory = stream->memory;
     FT_Error   error  = FT_Err_Ok;
 
     FT_UNUSED( error );
 
 
-    if ( delta_cnt > size )
-    {
-      FT_TRACE1(( "ft_var_readpackeddeltas: number of points too large\n" ));
-      return NULL;
-    }
-
     if ( FT_NEW_ARRAY( deltas, delta_cnt ) )
       return NULL;
 
-    i = 0;
-    while ( i < delta_cnt )
+    i          = 0;
+    bytes_used = 0;
+
+    while ( i < delta_cnt && bytes_used < size )
     {
       runcnt = FT_GET_BYTE();
       cnt    = runcnt & GX_DT_DELTA_RUN_COUNT_MASK;
 
+      bytes_used++;
+
       if ( runcnt & GX_DT_DELTAS_ARE_ZERO )
       {
-        /* `runcnt' zeroes get added */
+        /* `cnt` + 1 zeroes get added */
         for ( j = 0; j <= cnt && i < delta_cnt; j++ )
           deltas[i++] = 0;
       }
       else if ( runcnt & GX_DT_DELTAS_ARE_WORDS )
       {
-        /* `runcnt' shorts from the stack */
+        /* `cnt` + 1 shorts from the stack */
+        bytes_used += 2 * ( cnt + 1 );
+        if ( bytes_used > size )
+        {
+          FT_TRACE1(( "ft_var_readpackeddeltas:"
+                      " number of short deltas too large\n" ));
+          goto Fail;
+        }
+
         for ( j = 0; j <= cnt && i < delta_cnt; j++ )
           deltas[i++] = FT_intToFixed( FT_GET_SHORT() );
       }
       else
       {
-        /* `runcnt' signed bytes from the stack */
+        /* `cnt` + 1 signed bytes from the stack */
+        bytes_used += cnt + 1;
+        if ( bytes_used > size )
+        {
+          FT_TRACE1(( "ft_var_readpackeddeltas:"
+                      " number of byte deltas too large\n" ));
+          goto Fail;
+        }
+
         for ( j = 0; j <= cnt && i < delta_cnt; j++ )
           deltas[i++] = FT_intToFixed( FT_GET_CHAR() );
       }
 
       if ( j <= cnt )
       {
-        /* bad format */
-        FT_FREE( deltas );
-        return NULL;
+        FT_TRACE1(( "ft_var_readpackeddeltas:"
+                    " number of deltas too large\n" ));
+        goto Fail;
       }
     }
 
+    if ( i < delta_cnt )
+    {
+      FT_TRACE1(( "ft_var_readpackeddeltas: not enough deltas\n" ));
+      goto Fail;
+    }
+
     return deltas;
+
+  Fail:
+    FT_FREE( deltas );
+    return NULL;
   }