fix: generate an error on prefix with empty local name

A case like <foo bar:="1"/> used to pass. It is not wellformed due to the prefix
(bar) lacking a local name. It no longer passes.

Closes lddubeau#5.

lib/saxes.js

@@ -1658,20 +1658,13 @@ class SaxesParser {
       return { prefix: "", local: name };
     }
 
-    // A colon at the start of the name is illegal.
-    if (colon === 0) {
-      this.fail(`malformed name: ${name}.`);
-    }
-
     const local = name.substring(colon + 1);
-    if (local.indexOf(":") !== -1) {
+    const prefix = name.substring(0, colon);
+    if (prefix === "" || local === "" || local.indexOf(":") !== -1) {
       this.fail(`malformed name: ${name}.`);
     }
 
-    return {
-      prefix: name.substring(0, colon),
-      local,
-    };
+    return { prefix, local };
   }
 
   /** @private */