Avoid av_memcpy_backptr hang without extra branch.

This only happens for a "back" value of 0 which is invalid anyway,
but lcldec does not properly validate input.
Also extend the documentation to specify valid values.

Signed-off-by: Reimar Döffinger <Reimar.Doeffinger@gmx.de>

libavutil/lzo.h

@@ -62,11 +62,14 @@ int av_lzo1x_decode(void *out, int *outlen, const void *in, int *inlen);
 /**
  * @brief deliberately overlapping memcpy implementation
  * @param dst destination buffer; must be padded with 12 additional bytes
- * @param back how many bytes back we start (the initial size of the overlapping window)
+ * @param back how many bytes back we start (the initial size of the overlapping window), must be > 0
  * @param cnt number of bytes to copy, must be >= 0
  *
  * cnt > back is valid, this will copy the bytes we just copied,
  * thus creating a repeating pattern with a period length of back.
+ * Note that lcldec currently can set back == 0 - which is wrong and
+ * makes no sense, but the code should at least avoid crashing or hanging
+ * for this case.
  */
 void av_memcpy_backptr(uint8_t *dst, int back, int cnt);