avutil/random_seed: add av_random_bytes()

Uses the existing code for av_get_random_seed() to return a buffer with
cryptographically secure random data, or an error if none could be generated.

Signed-off-by: James Almer <jamrial@gmail.com>

libavutil/random_seed.h

@@ -36,6 +36,19 @@
  */
 uint32_t av_get_random_seed(void);
 
+/**
+ * Generate cryptographically secure random data, i.e. suitable for use as
+ * encryption keys and similar.
+ *
+ * @param buf buffer into which the random data will be written
+ * @param len size of buf in bytes
+ *
+ * @retval 0                         success, len bytes of random data was written
+ *                                   into buf
+ * @retval "a negative AVERROR code" random data could not be generated
+ */
+int av_random_bytes(uint8_t *buf, size_t len);
+
 /**
  * @}
  */