mod/transport: Perform null check before dereferencing 'buffer' pointer

In the function transport_transmit, dereferencing of the 'buffer' pointer to access member variables was occurring before the check to ensure the pointer was not null.

In the function transport_respond a call is made to fwk_str_memcpy passing the 'buffer' pointer as a parameter was made before the check to ensure the pointer was not null.

This did not cause an issue as there is no code path that leads to these functions being called with ‘buffer’ set to null.

Signed-off-by: Matthew Wellings <matthew.wellings@arm.com>