Fix MSVC 2015 compiler complaint

This is/was slowly, but surely, driving me insane:

DM.cpp(150): warning C4838: conversion from 'DWORD' to 'int' requires a narrowing conversion

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Review URL: https://codereview.chromium.org/1749363002

dm/DM.cpp

@@ -138,7 +138,7 @@ static std::atomic<bool> in_signal_handler{false};
     static LONG WINAPI handler(EXCEPTION_POINTERS* e) {
         static const struct {
             const char* name;
-            int code;
+            DWORD code;
         } kExceptions[] = {
         #define _(E) {#E, E}
             _(EXCEPTION_ACCESS_VIOLATION),