update infra

* submodules/infra 15bea2d...6898254 (3):
  > Merge into master from pull request #15: AIM: add a debug-only assert macro (https://github.com/floodlight/infra/pull/15)
  > Merge into master from pull request #14: clean up clang static analysis warnings (https://github.com/floodlight/infra/pull/14)
  > Merge into master from pull request #13: AIM: do not fail if allocating zero bytes returns NULL (https://github.com/floodlight/infra/pull/13)
clean up static analysis warnings
Merge into master from pull request #13:
clean up static analysis warnings (https://github.com/floodlight/bigcode/pull/13)