invoke_swaybar: fix message length header size
invoke_swaybar: fix message length header size

size_t/ssize_t are 8 bytes on 64bit systems, so use the proper size to
transmit that information.
This could lead to ridiculously large alloc as len is not initialized to zero

Found through static analysis

taiyu-len/sway,1ace/sway,SirCmpwn/sway,1ace/sway,ascent12/sway,taiyu-len/sway,ascent12/sway,taiyu-len/sway,1ace/sway,ascent12/sway