bug#42822: Build of master db3a7df fails with MINGW64 console of MSYS2

[Yasuhiro KIMURA]

I checked out master db3a7df and built it with MINGW64 console of
MSYS2 on 64bit Windows 10 version 1909. Then I got following error.

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  CC       search.o
  CC       regex-emacs.o
  CC       undo.o
  CC       alloc.o
  CC       pdumper.o
pdumper.c: In function 'dump_read_all':
pdumper.c:5078:60: error: conversion from 'size_t' {aka 'long long unsigned 
int'} to 'unsigned int' may change value [-Werror=conversion]
 5078 |       ssize_t chunk = read (fd, (char *) buf + bytes_read, 
chunk_to_read);
      |                                                            ^~~~~~~~~~~~~
cc1.exe: some warnings being treated as errors
make[1]: *** [Makefile:401: pdumper.o] Error 1
make[1]: Leaving directory '/c/Users/yasu/Work/Emacs/master/emacs/src'
make: *** [Makefile:424: src] Error 2
(MINGW64)yasu@rolling[1188]% 
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According to the result of git bisect, following commit is the cause
of the problem.

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commit 9e4e4775364490d31406f247237934dd7f203808 (HEAD)
Author: Paul Eggert <eggert@cs.ucla.edu>
Date:   Tue Aug 11 02:16:54 2020 -0700

    In pdumper, simplify INT_MAX computation

    * src/pdumper.c (dump_read_all): Avoid unnecessary cast.
    Also, round down to page size, as sysdep.c does.
    Also, don t assume INT_MAX <= UINT_MAX (!).
----------------------------------------------------------------------

If I go back to master db3a7df and revert this commit, then build
completes successfully.

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Yasuhiro KIMURA