vim-patch:7.4.1742

Problem:    strgetchar() does not work correctly.
Solution:   use mb_cptr2len().  Add a test. (Naruhiko Nishino)

5d18e0eca5

src/nvim/eval.c

@@ -15640,7 +15640,7 @@ static void f_strgetchar(typval_T *argvars, typval_T *rettv, FunPtr fptr)
         break;
       }
       charidx--;
-      byteidx += mb_char2len(str[byteidx]);
+      byteidx += mb_cptr2len(str + byteidx);
     }
   }
 }