win: fix 'potentially uninitialized local variable' warnings
win: fix 'potentially uninitialized local variable' warnings

e.g.

d:\src\crashpad\crashpad\minidump\minidump_memory_writer_test.cc(347) : error C2220: warning treated as error - no 'object' file generated
d:\src\crashpad\crashpad\minidump\minidump_memory_writer_test.cc(347) : warning C4701: potentially uninitialized local variable 'memory_list' used
d:\src\crashpad\crashpad\minidump\minidump_memory_writer_test.cc(347) : warning C4703: potentially uninitialized local pointer variable 'memory_list' used

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

Review URL: https://codereview.chromium.org/899793002

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