Allocate enough data in buffer to hold an RTP packet with csrc_count CSRCs, a payload length of
payload_len and padding of pad_len. MALLOCDATA of buffer will be overwritten and will not be freed. All
other RTP header fields will be set to 0/FALSE.
| buffer | a Buffer |
| pad_len | the amount of padding |
| payload_len | the length of the payload |
| csrc_count | the number of CSRC entries |