Fix memleak when open encoder failed

Fix #75

I think this should be enough: CHECKED_PARAM_STRDUP() calls x264_param_strdup() which adds the allocated string pointer to param->opaque, interpreted as a strdup_buffer. So all strings there were allocated (and not literals, for instance) are free'd when you call x264_param_cleanup().

I've tested this with valgrind, and it seems to solve the leak.

Since it's a rather trivial fix, can somebody please merge this?