Fix crash in net_wm_state_is_cb when running GCC ASan-enabled build

Check size of X11 property before dereferencing as an Atom.
This fixes:
==10203==ERROR: AddressSanitizer: heap-buffer-overflow on address
0x602000065970 at pc 0x5c5834fb8812 bp 0x7819c717c220 sp 0x7819c717c218
READ of size 8 at 0x602000065970 thread T2 (X11 Events)
    #0 0x5c5834fb8811 in net_wm_state_is_cb ../conio/x_events.c:1541

But I'm not clear when nitems_return (nir) might represent > 8-bit items or how
to detect that, so perhaps there's a *better* fix?

src/conio/x_events.c

@@ -1538,7 +1538,7 @@ net_wm_state_is_cb(bool (*cb)(Atom))
 				break;
 			}
 			ret = (Atom *)prop;
-			if (cb(*ret))
+			if (nir == sizeof(Atom) && cb(*ret))
 				is = true;
 			x11.XFree(prop);
 			if (is)