bgpd: Ensure FRR has enough data to read 2 bytes in peek_for_as4_capability

bgpd: Ensure FRR has enough data to read 2 bytes in peek_for_as4_capability

In peek_for_as4_capability the code is checking that the
stream has at least 2 bytes to read ( the opt_type and the
opt_length ).  However if BGP_OPEN_EXT_OPT_PARAMS_CAPABLE(peer)
is configured then FRR is reading 3 bytes.  Which is not good
since the packet could be badly formated.  Ensure that
FRR has the appropriate data length to read the data.

Signed-off-by: Donald Sharp <sharpd@nvidia.com>