app/testpmd: fix allocation check in MTR parsing
app/testpmd: fix allocation check in MTR parsing

Malloc() function might return NULL due to insufficient space. Therefore,
check for handling memory allocation failure is added.

Coverity issue: 257039
Fixes: e63b50162aa3 ("app/testpmd: clean metering and policing commands")

Signed-off-by: Jasvinder Singh <97b766ecf460b59e2781d4fc0469065206870f5c@intel.com>

john-mcnamara-intel/dpdk,john-mcnamara-intel/dpdk,john-mcnamara-intel/dpdk,john-mcnamara-intel/dpdk