util: fix undefined behavior in wl_array_for_each

If a wl_array has size zero, wl_array_for_each computes NULL + 0 to get
to the end pointer. This should be fine, and indeed it would be fine in
C++. But the C specification has a mistake here and it is actually
undefined behavior. See
https://davidben.net/2024/01/15/empty-slices.html

Clang's -fsanitize=undefined flags this. I ran into this in Chromium's
build with wayland-scanner on one of our XML files.

../../third_party/wayland/src/src/scanner.c:1853:2: runtime error: applying zero offset to null pointer
    #0 0x55c979b8e02c in emit_code third_party/wayland/src/src/scanner.c:1853:2
    #1 0x55c979b89323 in main third_party/wayland/src/src/scanner.c
    #2 0x7f8dfdb8c6c9 in __libc_start_call_main csu/../sysdeps/nptl/libc_start_call_main.h:58:16
    #3 0x7f8dfdb8c784 in __libc_start_main csu/../csu/libc-start.c:360:3
    #4 0x55c979b70f39 in _start (...)

An empty XML file is sufficient to hit this case, so I've added it as a
test. To reproduce, undo the fix and include only the test, then build
with:

  CC=clang CFLAGS="-fno-sanitize-recover=undefined" meson build/ -Db_sanitize=undefined -Db_lundef=false
  ninja -C build test

Signed-off-by: David Benjamin <davidben@google.com>

src/wayland-util.h

@@ -598,6 +598,7 @@ wl_array_copy(struct wl_array *array, struct wl_array *source);
  */
 #define wl_array_for_each(pos, array)					\
 	for (pos = (array)->data;					\
+	     (array)->size != 0 &&					\
 	     (const char *) pos < ((const char *) (array)->data + (array)->size); \
 	     (pos)++)