throw error if deriving an invalid path string

...the path consists "m", numbers, /, and ' characters

lib/bip32.js

@@ -215,10 +215,13 @@ BIP32.prototype.derive = function(path) {
     var c = e[i];
 
     if (i == 0) {
-      if (c != 'm') throw new Error('invalid path');
+      if (c != 'm') throw new Error('bip32: invalid path');
       continue;
     }
 
+    if (parseInt(c.replace("'", "")).toString() !== c.replace("'", ""))
+      throw new Error('bip32: invalid path');
+
     var usePrivate = (c.length > 1) && (c[c.length - 1] == '\'');
     var childIndex = parseInt(usePrivate ? c.slice(0, c.length - 1) : c) & 0x7fffffff;