* c-common.c (format_wanted_type): Add reading_from_flag.

	* c-common.c (format_wanted_type): Add reading_from_flag.
	(print_char_table): Mark %s and %S formats with flag "R".
	(check_format_info_main): Set up reading_from_flag appropriately.
	If aflag, always set writing_in_flag rather than relying on the
	format used being a scanf format and so having it set.
	(check_format_types): Check for formats reading through null
	pointers.

testsuite:
	* gcc.dg/c90-printf-1.c: Add test for printf formats reading
	through a null pointer.

git-svn-id: svn+ssh://gcc.gnu.org/svn/gcc/trunk@38104 138bc75d-0d04-0410-961f-82ee72b054a4