tx: reduce memset

[AlexanderYastrebov]

• zero only the unused rest of the block
• use header pointer to eliminate memcpy

[AlexanderYastrebov]

I think there is also a potential to eliminate this mem copy:

wfb-ng/src/tx.cpp

Lines 178 to 179 in 2178edb

	// FEC data
	memcpy(p, buf, size);

by prepending encryption and session key buffers with radio headers but it would require changing Transmitter interface (inject_packet method).

[svpcom]

I think this will not increase speed significantly but reduce code readability a lot. Anyway cpu usage even on armv6 (rpi zero) is very low.

I think there is also a potential to eliminate this mem copy:

wfb-ng/src/tx.cpp

Lines 178 to 179 in 2178edb

	// FEC data
	memcpy(p, buf, size);

by prepending encryption and session key buffers with radio headers but it would require changing Transmitter interface (inject_packet method).

[AlexanderYastrebov]

I think there is also a potential to eliminate this mem copy

See #329