[core] SRTO_CONGESTION: Check optlen in getsockopt

[maxsharabayko]

Return an error if optlen passed in srt_getsockopt(..) for SRTO_CONGESTION is not big enough to fit the string value of an option (effectively four chars + one null-terminating character).

[maxsharabayko]

There is one tricky part here.
Both SRTO_CONGESTION and SRTO_STREAMID expect the buffer with extra space for a zero-terminating character.
This should be described in docs.

Example

char cctype[5];
int optlen = sizeof cctype;
srt_getsockopt(sock, 0, SRTO_CONGESTION, (void*) cctype, &optlen );

// Now cctype is: "live\0", optlen is 4.

This will fail due to small buffer provided:

char cctype[4];
int optlen = sizeof cctype;
srt_getsockopt(sock, 0, SRTO_CONGESTION, (void*) cctype, &optlen ); // will return SRT_ERROR