iface-modem-firmware: Initialize context to null
Without setting memory to NULL it is possible that ctx->list point to an unitialized pointer and trig a segfault when we free it (for example when the firmware list is not supported)
I came into this when issuing mmcli -m any --firmware-list with a not capable modem.
Signed-off-by: Frederic Martinsons frederic.martinsons@sigfox.com