tests: Handle -Wanalyzer-possible-null-dereference in damage/primitives.c

Tell the compiler not to warn us that malloc could possibly return NULL
in this unit test.

Reported in #1817:
xwayland-24.1.6/redhat-linux-build/../test/damage/primitives.c:97:13:
 warning[-Wanalyzer-possible-null-dereference]: dereference of possibly-NULL
 ‘get_image(setup, *setup.d) + (long unsigned int)i * 4’
xwayland-24.1.6/redhat-linux-build/../test/damage/primitives.c:97:27:
 warning[-Wanalyzer-possible-null-dereference]: dereference of possibly-NULL
 ‘setup.start_drawable_contents’

Fixes: 89901e14d ("test: Add the start of a testsuite for damage.")
Signed-off-by: Alan Coopersmith <alan.coopersmith@oracle.com>
Part-of: <https://gitlab.freedesktop.org/xorg/xserver/-/merge_requests/2167>

test/damage/primitives.c

@@ -77,6 +77,7 @@ get_image(struct test_setup *setup, xcb_drawable_t drawable)
 
     uint32_t *result = malloc(sizeof(uint32_t) *
                               setup->width * setup->height);
+    assert(result != NULL);
     memcpy(result, data, len);
     free(reply);