Patch to check for formats reading through null pointer

[Jeffrey A Law]

  In message <Pine.LNX.4.30.0012070031120.23840-100000@kern.srcf.societies.cam.
ac.uk>you write:
  > This patch adds a minor format checking feature: as well as the
  > existing checks for formats writing into constant objects or through
  > null pointers, this patch adds a check for formats reading through
  > null pointers.  It also cleans up the logic for these checks with the
  > GNU scanf %a flag (which happened to work anyway since all formats
  > taking %a did have the "W" flag).
  > 
  > Bootstrapped with no regressions on i686-pc-linux-gnu.  OK to commit?
  > 
  > gcc/ChangeLog:
  > 2000-12-07  Joseph S. Myers  <jsm28@cam.ac.uk>
  > 
  > 	* 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.
  > 
  > gcc/testsuite/ChangeLog:
  > 2000-12-07  Joseph S. Myers  <jsm28@cam.ac.uk>
  > 
  > 	* gcc.dg/c90-printf-1.c: Add test for printf formats reading
  > 	through a null pointer.
Approved.  Please install.

Thanks,
jeff