Prohibit default construction of function_output_iterator on function pointers.

Default-constructed function_output_iterator with function pointers is unusable
and previously would have contained an uninitialized function pointer.
Disable the default constructor using SFINAE to prevent misuse.

Also reformat code.

test/Jamfile.v2

@@ -58,6 +58,7 @@ test-suite iterator
     [ run function_input_iterator_test.cpp ]
     [ run function_output_iterator_test.cpp ]
     [ compile-fail function_output_iterator_cf.cpp ]
+    [ compile-fail function_output_iterator_def_ctor_cf.cpp ]
 
     [ run generator_iterator_test.cpp ]
 

test/function_output_iterator_def_ctor_cf.cpp

@@ -0,0 +1,14 @@
+// Copyright 2025 (c) Andrey Semashev
+// Distributed under the Boost Software License Version 1.0.
+// (See accompanying file LICENSE_1_0.txt or copy at
+// http://www.boost.org/LICENSE_1_0.txt)
+
+#include <boost/iterator/function_output_iterator.hpp>
+
+int main()
+{
+    boost::iterators::function_output_iterator< void (*)(int) > it;
+    (void)it;
+
+    return 0;
+}