warnings in win32 builds from trunk

[Corwin Brust]

Attempting to build a snapshot from the master branch today I noticed
some warnings.  Here they are; please let me know if there's else I
might helpfully do to troubleshoot/debug these (e.g. open a bug
report).

Maybe easier to read:
  https://bpa.st/6EBA

Inline (to EOM):

  CC       w32notify.o
In file included from C:/Users/corwi/emacs-build/git/master/src/process.c:33:
C:/Users/corwi/emacs-build/git/master/src/process.c: In function
'Fmake_process':
C:/Users/corwi/emacs-build/git/master/src/lisp.h:1649:31: warning:
null pointer dereference [-Wnull-dereference]
 1649 |   return XSTRING (string)->u.s.data;
      |          ~~~~~~~~~~~~~~~~~~~~~^~~~~
  CC       profiler.o
  CC       decompress.o
  CC       thread.o
  CC       systhread.o
  CC       sqlite.o
  CC       hbfont.o
  CC       w32fns.o
  CC       w32menu.o
  CC       w32reg.o
  CC       w32font.o
  CC       w32term.o
  CC       w32xfns.o
  CC       w32select.o
  CC       w32uniscribe.o
  CC       w32cygwinx.o
  CC       w32.o
  CC       w32console.o
  CC       w32heap.o
  CC       w32inevt.o
  CC       w32proc.o
  CC       w32image.o
  CC       fontset.o
  CC       fringe.o
  CC       image.o
C:/Users/corwi/emacs-build/git/master/src/w32menu.c: In function
'set_frame_menubar':
C:/Users/corwi/emacs-build/git/master/src/w32menu.c:324:9: warning:
'memcpy' offset [3, 10] from the object at '<unknown>' is out of the
bounds of referenced subobject 'contents' with type 'struct Lisp_X
*[]' at offset 3 [-Warray-bounds]
  324 |         memcpy (previous_items, XVECTOR (f->menu_bar_vector)->contents,
      |         ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
  325 |                 previous_menu_items_used * word_size);
      |                 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
In file included from C:/Users/corwi/emacs-build/git/master/src/w32menu.c:26:
C:/Users/corwi/emacs-build/git/master/src/lisp.h:1725:17: note:
subobject 'contents' declared here
 1725 |     Lisp_Object contents[FLEXIBLE_ARRAY_MEMBER];
      |                 ^~~~~~~~
C:/Users/corwi/emacs-build/git/master/src/w32heap.c: In function 'getrlimit':
C:/Users/corwi/emacs-build/git/master/src/w32heap.c:853:14: warning:
'm' may be used uninitialized [-Wmaybe-uninitialized]
  853 |         if (!VirtualQuery ((LPCVOID) &m, &m, sizeof m))
      |              ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
In file included from C:/msys2/mingw64/x86_64-w64-mingw32/include/winbase.h:25,
                 from C:/msys2/mingw64/x86_64-w64-mingw32/include/windows.h:70,
                 from C:/Users/corwi/emacs-build/git/master/src/w32common.h:24,
                 from C:/Users/corwi/emacs-build/git/master/src/w32heap.c:54:
C:/msys2/mingw64/x86_64-w64-mingw32/include/memoryapi.h:45:28: note:
by argument 1 of type 'LPCVOID' {aka 'const void *'} to 'VirtualQuery'
declared here
   45 |   WINBASEAPI SIZE_T WINAPI VirtualQuery (LPCVOID lpAddress,
PMEMORY_BASIC_INFORMATION lpBuffer, SIZE_T dwLength);
      |                            ^~~~~~~~~~~~
C:/Users/corwi/emacs-build/git/master/src/w32heap.c:844:34: note: 'm'
declared here
  844 |         MEMORY_BASIC_INFORMATION m;
      |                                  ^
  CC       json.o
  CC       tparam.o