Fix: flt.utils.muxer: Explicit null dereferenced

Fix: flt.utils.muxer: Explicit null dereferenced

Fixes coverity #1404006

  ID 1404006 (#1 of 1): Explicit null dereferenced (FORWARD_NULL)
  18. var_deref_op: Dereferencing null pointer *muxer_upstream_msg_iter

Signed-off-by: Francis Deslauriers <francis.deslauriers@efficios.com>
Change-Id: I055200226ab13653b219a084f38a2dd9bb16664b
Reviewed-on: https://review.lttng.org/c/babeltrace/+/1859
CI-Build: Philippe Proulx <eeppeliteloop@gmail.com>
Tested-by: jenkins <jenkins@lttng.org>
Reviewed-by: Philippe Proulx <eeppeliteloop@gmail.com>