MINOR: connection: Add function to get src/dst without updating the connection

MINOR: connection: Add function to get src/dst without updating the connection

conn_get_src() and conn_get_dst() functions are used to fill the source and
destination addresses of a connection. On success, ->src and ->dst
connection fields can be safely used.

For convenience, 2 new functions are added here: conn_src() and conn_dst().
These functions return the corresponding address, as a const and only if it
is already set. Otherwise NULL is returned.