net/mlx5: fix GCC uninitialized variable warning

net/mlx5: fix GCC uninitialized variable warning

When building with -Db_sanitize=thread, GCC gives a warning:

drivers/net/mlx5/mlx5_flow_meter.c: In function ‘mlx5_flow_meter_create’:
drivers/net/mlx5/mlx5_flow_meter.c:1170:33: warning: ‘legacy_fm’ may be
    used uninitialized in this function [-Wmaybe-uninitialized]

This is a false-positive: legacy_fm is initialized and used
if and only if priv->sh->meter_aso_en is false.
Work around this by initializing legacy_fm to NULL.
Add an assertion before legacy_fm use in case the logic changes.

Fixes: 444320186393 ("net/mlx5: support meter creation with policy")
Cc: stable@dpdk.org
Reported-by: Stephen Hemminger <stephen@networkplumber.org>
Signed-off-by: Dmitry Kozlyuk <dkozlyuk@nvidia.com>
Acked-by: Matan Azrad <matan@nvidia.com>