Ensure we don't modify the original PDF data when sanitizing TrueType

src/fonts.js

@@ -3870,6 +3870,9 @@ var Font = (function FontClosure() {
         }
       }
 
+      // The following steps modify the original font data, making copy
+      font = new Stream(new Uint8Array(font.getBytes()));
+
       // Check that required tables are present
       var requiredTables = ['OS/2', 'cmap', 'head', 'hhea',
                              'hmtx', 'maxp', 'name', 'post'];