Tweak windows.UNWIND_HISTORY_TABLE for mem.zeroes compatibility

`mem.zeroes` was not compatible with the structure because it contained
non-nullable pointers. Those pointers have been made nullable.

`history_table` is also now initialized to undefined. It is an output
parameter of `windows.ntdll.RtlLookupFunctionEntry` and never read
afterward, so there should be no functional change.

Author: jayschwa

lib/std/debug.zig

@@ -764,7 +764,7 @@ pub noinline fn walkStackWindows(addresses: []usize, existing_context: ?*const w
 
     var i: usize = 0;
     var image_base: usize = undefined;
-    var history_table: windows.UNWIND_HISTORY_TABLE = std.mem.zeroes(windows.UNWIND_HISTORY_TABLE);
+    var history_table: windows.UNWIND_HISTORY_TABLE = undefined;
 
     while (i < addresses.len) : (i += 1) {
         const current_regs = context.getRegs();

lib/std/os/windows.zig

@@ -4104,7 +4104,7 @@ pub const EXCEPTION_ROUTINE = *const fn (
 pub const UNWIND_HISTORY_TABLE_SIZE = 12;
 pub const UNWIND_HISTORY_TABLE_ENTRY = extern struct {
     ImageBase: ULONG64,
-    FunctionEntry: *Self.RUNTIME_FUNCTION,
+    FunctionEntry: ?*Self.RUNTIME_FUNCTION,
 };
 
 pub const UNWIND_HISTORY_TABLE = extern struct {