fix: convert numeric character references representing < and & into &lt; and &amp;, as otherwise may break well-formedness; fixes svg#1

Author: brettz9

lib/sax.js

@@ -827,6 +827,12 @@
         num = parseInt(entity, 10)
         numStr = num.toString(10)
       }
+      switch (num) {
+        case 60: // '<'
+          return '&lt;'
+        case 38: // '&'
+          return '&amp;'
+      }
     }
     entity = entity.replace(/^0+/, '')
     if (isNaN(num) || numStr.toLowerCase() !== entity) {