Adjust gtkutil.c to build with gcc (GCC) 11.2.1 with -Wanalyzer-null-argument

[Alexander Kuleshov]

Hello,

I'm getting:

gtkutil.c: In function ‘xg_item_label_same_p’:
gtkutil.c:2936:54: error: use of NULL where non-null expected
[CWE-476] [-Werror=analyzer-null-argument]
 2936 |                   && ((!old_label && !utf8_label) || strcmp
(utf8_label, old_label) == 0));

error during building of GNU Emacs from master on Fedora 34 with GCC
11.2.1. I've attached a simple fix
for that.

Thank you.

0001-Fix-building-with-gcc-11.1.2-and-Wanalyzer-null-argu.patch
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