string_bytes: add StringBytes::IsValidString()

Performs a quick, non-exhaustive check on the input string to see if
it's compatible with the specified string encoding.

Curently it only checks that hex strings have a length that is a
multiple of two.

src/string_bytes.cc

@@ -257,6 +257,13 @@ size_t StringBytes::Write(char* buf,
 }
 
 
+bool StringBytes::IsValidString(Handle<String> string, enum encoding enc) {
+  if (enc == HEX && string->Length() % 2 != 0) return false;
+  // TODO(bnoordhuis) Add BASE64 check?
+  return true;
+}
+
+
 // Quick and dirty size calculation
 // Will always be at least big enough, but may have some extra
 // UTF8 can be as much as 3x the size, Base64 can have 1-2 extra bytes