common/mlx5: fix missing validation in devargs parsing

common/mlx5: fix missing validation in devargs parsing

The rte_kvargs_parse function parses the arguments
"key=value,key=value,..." string and return an allocated structure that
contains a key/value list.
It enables also to send a key without value and updates the values in
the following ways:
 - "key=value,key,..." - value is updated as NULL.
 - "key=value,key=,..." - value is updated as "" (empty string).

Mlx5 PMDs use this function to parse, but they don't support key without
value. They send the value as an argument to strtol function.
When strtol gets NULL as a parameter it cause a crash, when it gets ""
(empty string) it returns 0.

Adds a check that will prevent an argument in these formats, and returns
an error for it.

Fixes: 85209924039c ("common/mlx5: share memory related devargs")
Cc: stable@dpdk.org
Signed-off-by: Michael Baum <michaelba@nvidia.com>
Acked-by: Matan Azrad <matan@nvidia.com>