[1.x] Replace g_malloc () with g_malloc0 () in the code

[fengyj2022]

g_malloc () in the code should be replaced with g_malloc0 () to simplify data initialization@ lminiero

[lminiero]

We use g_malloc on purpose in many places, see here for context: https://github.com/meetecho/janus-gateway/pull/2575#issuecomment-790653240