BUG报告，void SrsRtpFrameBuffer::ShiftSubsequentPackets 如果packets_为空会出现异常

[vicent-q]

修正如下：

void SrsRtpFrameBuffer::ShiftSubsequentPackets(PacketIterator it,
int steps_to_shift)
{
if (it != packets_.end()) {
++it;
}
if (it == packets_.end()) {
return;
}

uint8_t* first_packet_ptr = const_cast<uint8_t*>((*it).dataPtr);
int shift_length = 0;

// Calculate the total move length and move the data pointers in advance.
for (; it != packets_.end(); ++it) {
    shift_length += (*it).sizeBytes;

    if ((*it).dataPtr != NULL) {
        (*it).dataPtr += steps_to_shift;
    }
}

memmove(first_packet_ptr + steps_to_shift, first_packet_ptr, shift_length);

}