net/softnic: fix null dereference in arguments parsing

net/softnic: fix null dereference in arguments parsing

When there is no "firmware" in arguments, the "firmware" pointer is
null, and will be dereferenced by rte_strscpy().

This patch moves the code block which copies character string from
"firmware" to "p->firmware" into the "if" statements where "firmware"
argument exists and it is duplicated successfully.

Coverity issue: 372136
Fixes: d8f852f5f369 ("net/softnic: fix memory leak in arguments parsing")
Cc: stable@dpdk.org
Signed-off-by: Dapeng Yu <dapengx.yu@intel.com>
Acked-by: Jasvinder Singh <jasvinder.singh@intel.com>