Buffer overflow in _gnutls_bin2hex()

Maybe this can never be reached, but better be cautious.

If oldlen == 0, then buffer overflow.

@@ -454,13 +459,15 @@ char *_gnutls_bin2hex(const void *_old, size_t oldlen, char *buffer,
        }
 
        i = j = 0;
-       sprintf(&buffer[j], "%.2x", old[i]);
-       j += 2;
-       i++;
-
-       for (; i < oldlen && j + step < buffer_size; j += step) {
-               sprintf(&buffer[j], "%s%.2x", separator, old[i]);
+       if (oldlen > 0) {
+               sprintf(&buffer[j], "%.2x", old[i]);
+               j += 2;
                i++;
+
+               for (; i < oldlen && j + step < buffer_size; j += step) {
+                       sprintf(&buffer[j], "%s%.2x", separator, old[i]);
+                       i++;
+               }
        }
        buffer[j] = '\0';