Fix bad usage of strtol()

Fix bad usage of strtol()

Someone didn't RTFM.

1. Error code wasn't being catched.
2. Second argument was NULL, so the caller had no way of knowing
   whether the string was fully consumed.
2. The code wasn't making sure the input was a clean hex string.
3. The input string wasn't NULL-terminated.

In the end, I ditched strtol(). It's too dangerous and cluttered for
this purpose.