GB28181: Fix memory overlap for small packets.

[winlinvip]

In a certain scenario, when the package is relatively small and there is overlapping in the reserved area, memcpy should not be used, but memmove should be used instead.

srs_error_t SrsLazyGbMediaTcpConn::do_cycle() {
    uint32_t reserved = 0;
    for (;;) {
        uint16_t length = 0; // If it's 30
        if ((err = conn_->read_fully(buffer_ + reserved, length, NULL)) != srs_success) {

        reserved = b.left(); // If it's 20
        if (reserved) {
            b.read_bytes((char*)buffer_, reserved); // Crash here.
            pack_->media_reserved_++;
        }

When this situation occurs, b.read_bytes is actually equivalent to memcpy, which copies the last 20 bytes of b (30 bytes) to the beginning, resulting in overlap. It is equivalent to the following situation:

int length = 30;
char buffer[length];

char* p = buffer + 10;
int reserved = 20;
memcpy(buffer, p, reserved);

Solution: Changing it to memmove will solve the issue and support overlap.

Once again, thanks to @chen-guanghua for introducing the asan tool, which helped uncover many potential issues.

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