Update libsrtp to 1.5.2

srtp/test/rtp.c

 /*
  * rtp.c
  *
  * library functions for the real-time transport protocol
  *
  * David A. McGrew
  * Cisco Systems, Inc.
  */
 
 /*
@@ skipping 205 matching lines @@
   free(rtp_ctx);
 }
 
 rtp_receiver_t 
 rtp_receiver_alloc(void) {
   return (rtp_receiver_t)malloc(sizeof(rtp_receiver_ctx_t));
 }
 
 void
 rtp_receiver_dealloc(rtp_receiver_t rtp_ctx) {
-  return free(rtp_ctx);
+  free(rtp_ctx);
 }