win: warning fixes in minidump_thread_writer_test.cc
win: warning fixes in minidump_thread_writer_test.cc

- unaligned due to heap allocation
- potentially uninitialized local
- truncation of uint64_t -> DWORD

R=mark@chromium.org
BUG=crashpad:1

Review URL: https://codereview.chromium.org/903603002

hokein/crashpad,hokein/crashpad,chromium/crashpad,Chilledheart/crashpad,hankbao/Crashpad,Chilledheart/crashpad,atom/crashpad,chromium/crashpad,Chilledheart/crashpad,atom/crashpad,atom/crashpad,hankbao/Crashpad,hokein/crashpad,chromium/crashpad,hankbao/Crashpad