realloc/g_realloc() allocates memory if given ptr is NULL.

Note:
This changes a bit the code, since now sb_cc is always reset to 0,
even if old and new value are the same. This seems more coherent, but
may have weird side-effects if code relies on it.

Reviewing usage of sbreserve() reveals that it is used before the
socket buffer receives any data, at tcp_input() socket creation time,
and during tcp_mss() which is earlier in TCP socket state.

Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>