Fix bug in list insertion:

The output iterator has its previous field incorrect.

m-list.h

@@ -503,6 +503,7 @@
     }                                                                         \
     /* Update insertion_point to this element */                              \
     insertion_point->current = next;                                          \
+    insertion_point->previous = current;                                      \
     M_L1ST_CONTRACT(list);                                                    \
   }                                                                           \
                                                                               \

tests/test-mlist.c

@@ -867,9 +867,36 @@ static void test_let_string(void)
     }
 }
 
+static void test_insert_remove(void)
+{
+  list_uint_t l;
+
+  list_uint_init(l);
+  list_uint_push_back(l, 1);
+  list_uint_push_back(l, 2);
+  list_uint_push_back(l, 3);
+
+  list_uint_it_t u;
+  unsigned w= 4;
+  list_uint_it(u, l);
+  list_uint_next(u);
+  assert ( list_uint_size(l) == 3);
+  list_uint_insert(l, u, w); 
+  assert ( list_uint_size(l) == 4);
+  list_uint_remove(l, u); 
+  assert ( list_uint_size(l) == 3);
+  list_uint_insert(l, u, w); 
+  assert ( list_uint_size(l) == 4);
+  list_uint_remove(l, u); 
+  assert ( list_uint_size(l) == 3);
+
+  list_uint_clear(l);
+}
+
 int main(void)
 {
   test_uint();
+  test_insert_remove();
   test_mpz();
   test_dual_push1();
   test_dual_it1();