vim-patch:9.1.1611: possible undefined behaviour in mb_decompose() (#35275)

Problem:  possible undefined behaviour in mb_decompose(), when using the
          same pointer as argument several times
Solution: use separate assignments to avoid reading and writing the same
          object at the same time (Áron Hárnási)

closes: vim/vim#17953

c43a0614d4

Co-authored-by: Áron Hárnási <aron.harnasi@gmail.com>
(cherry picked from commit 77500c5ad5)

src/nvim/regexp.c

@@ -1727,7 +1727,8 @@ static void mb_decompose(int c, int *c1, int *c2, int *c3)
     *c3 = d.c;
   } else {
     *c1 = c;
-    *c2 = *c3 = 0;
+    *c2 = 0;
+    *c3 = 0;
   }
 }