app/testpmd: fix stack overwriting by flow command

app/testpmd: fix stack overwriting by flow command

The parameter type parser function stores a stack-local address in the
global parser context when parsing a "prefix" keyword.

This usually translates to "Bad arguments" errors even for correct flow
rules as stack gets overwritten by subsequent function calls.

Fixes: d3f61b7bad20 ("app/testpmd: add flow item spec prefix length")
Cc: stable@dpdk.org
Signed-off-by: Adrien Mazarguil <adrien.mazarguil@6wind.com>