PR pretty-print/67567 do not pass NULL as a string

PR pretty-print/67567 do not pass NULL as a string

Fortran passes NULL where a non-null string is expected by the pretty-printer,
which causes a sanitizer warning. This could have been found earlier by using
gcc_checking_assert. Even if the assertion is false, the result is just an
incomplete diagnostic, thus it seems more user-friendly to assert only when
checking. I do not have any idea how to properly fix the Fortran bug, thus this
patch simply works-around it.

gcc/fortran/ChangeLog:

2015-09-25  Manuel López-Ibáñez  <manu@gcc.gnu.org>

	PR pretty-print/67567
	* resolve.c (resolve_fl_procedure): Work-around when iface->module
	== NULL.

gcc/ChangeLog:

2015-09-25  Manuel López-Ibáñez  <manu@gcc.gnu.org>

	PR pretty-print/67567
	* pretty-print.c (pp_string): Add gcc_checking_assert.
	* pretty-print.h (output_buffer_append_r): Likewise.

From-SVN: r228131