Fix issue in string_previous

m-string.h

@@ -1842,6 +1842,7 @@ m_string_previous(m_string_it_t it)
       bool b = m_string_end_p(it);
       M_ASSERT(!b);
       (void) b;
+      return;
     }
   }
   /* We reach the start of the string: mark the iterator to the end */

tests/test-mstring.c

@@ -119,6 +119,7 @@ static void test_utf8_it(void)
     i--;
     assert (string_get_cref(it) == tab[i]);
   }
+  assert(i == 0);
 
   string_set_str (s, "H€llo René Chaînôr¬");
   assert(string_length_u(s) == 19);