Warnings in mingw64 builds on master

[Andy Moreton]

Hi,

Building master for 64bit mingw64 gives the following warnings:

C:/emacs/git/emacs/master/src/w32reg.c:146:1: warning: function might be 
candidate for attribute 'malloc' [-Wsuggest-attribute=malloc]
  146 | w32_get_string_resource (void *v_rdb, const char *name, const char 
*class)
      | ^~~~~~~~~~~~~~~~~~~~~~~

C:/emacs/git/emacs/master/src/w32menu.c: In function 'set_frame_menubar':
C:/emacs/git/emacs/master/src/w32menu.c:326:2: warning: 'memcpy' offset [3, 10] 
from the object at '<unknown>' is out of the bounds of referenced subobject 
'contents' with type 'union Lisp_X *[]' at offset 3 [-Warray-bounds]
  326 |  memcpy (previous_items, XVECTOR (f->menu_bar_vector)->contents,
      |  ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
  327 |   previous_menu_items_used * word_size);
      |   ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
In file included from C:/emacs/git/emacs/master/src/w32menu.c:26:
C:/emacs/git/emacs/master/src/lisp.h:1631:17: note: subobject 'contents' 
declared here
 1631 |     Lisp_Object contents[FLEXIBLE_ARRAY_MEMBER];
      |                 ^~~~~~~~

C:/emacs/git/emacs/master/src/w32.c: In function '_sys_read_ahead':
C:/emacs/git/emacs/master/src/w32.c:8785:10: warning: 'rc' may be used 
uninitialized in this function [-Wmaybe-uninitialized]
 8785 |   if (rc == sizeof (char))
      |       ~~~^~~~~~~~~~~~~~~~
C:/emacs/git/emacs/master/src/image.c:116: warning: macro "PIX_MASK_DRAW" is 
not used [-Wunused-macros]
  116 | #define PIX_MASK_DRAW 1
      | 

These must have been introduced fairly recently, as the build was clean
earlier this year.

    AndyM