ortest: fix perl warnings in "make gen"

[agentzh]

Seeing the following this morning:

./parse-logs -o ortest/ortest_20190211-173752_ip-172-30-2-103.json ortest/ortest_20190211-173752_ip-172-30-2-103.log
linux x86_64
Use of uninitialized value in string eq at ./parse-logs line 873, <> line 44835.
tl-ngx_stream_lua	twv-ngx_stream_lua	twv-lua-resty-core	tw-ngx_lua	t-ngx_lua	th-ngx_lua	tl-lua-resty-core	thv-ngx_stream_lua	tr-ngx_stream_lua
./parse-logs -o ortest/ortest_20190211-173752_ip-172-30-2-106.json ortest/ortest_20190211-173752_ip-172-30-2-106.log
linux x86_64
Use of uninitialized value in string eq at ./parse-logs line 873, <> line 165637.