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)

Signed-off-by: Frederic Martinsons <frederic.martinsons@sigfox.com>

src/mm-iface-modem-firmware.c

@@ -160,7 +160,7 @@ handle_list (MmGdbusModemFirmware *skeleton,
 {
     HandleListContext *ctx;
 
-    ctx = g_slice_new (HandleListContext);
+    ctx = g_slice_new0 (HandleListContext);
     ctx->skeleton = g_object_ref (skeleton);
     ctx->invocation = g_object_ref (invocation);
     ctx->self = g_object_ref (self);