DPP: Fix compiler warning of testing code
../src/common/dpp.c: In function 'dpp_test_gen_invalid_key':
../src/common/dpp.c:5531:10: warning: return makes integer from pointer without a cast [-Wint-conversion]
return NULL;
^
Signed-off-by: Masashi Honma <masashi.honma@gmail.com>