e3pbltool: make process_command() void

Make process_command() void.
e3pbltool: writeblock(): initialize kbs

Initialize kbs, otherwise the value is garbage if nothing was transferred.
e3pbltool: process_command(): meminfo: return early on failure

Return early on failure to avoid an uninitialized variable being used.
e3pbltool: process_command(): meminfo: initialize reply packet to 0

Initialize reply packet to 0 to make behaviour more deterministic
in case the device returns short data.