bug_reporter.c: suppress warnings

* ext/-test-/bug_reporter/bug_reporter.c (sample_bug_reporter): cast
  via uintptr_t to suppress warning cast between pointer and different
  size integer by gcc 4.8.

* ext/-test-/bug_reporter/bug_reporter.c (register_sample_bug_reporter):
  ditto.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@43339 b2dd03c8-39d4-4d8f-98ff-823fe69b080e