app/crypto-perf: fix uninitialized variable

app/crypto-perf: fix uninitialized variable

This commit ixes problem with uninitialized nb_cryptodevs variable by
initialize it with 0 value. Program could jump to err label
without running cperf_initialize_cryptodev() function. Also assign 0
value to nb_cryptodevs after cperf_initialize_cryptodev() when value is
negative.

Coverity issue: 141073
Fixes: f8be1786b1b8 ("app/crypto-perf: introduce performance test application")

Signed-off-by: Aleksander Gajewski <aleksanderx.gajewski@intel.com>
Acked-by: Pablo de Lara <pablo.de.lara.guarch@intel.com>