getatom: fix potential uninitialized atom variable

The getatom function returns the atom variable, which is only
initialized in case of a success. This results in a maybe-uninitialized
warning/error. After this commit, now a zero value is returned in case
of error.

dwl.c

@@ -762,7 +762,7 @@ focusstack(const Arg *arg)
 Atom
 getatom(xcb_connection_t *xc, const char *name)
 {
-	Atom atom;
+	Atom atom = 0;
 	xcb_generic_error_t *error;
 	xcb_intern_atom_cookie_t cookie;
 	xcb_intern_atom_reply_t *reply;