PATCH for fortran/85735, f951 crash on empty input

[Marek Polacek]

Running
./f951 -quiet
^D
crashes in gfc_create_decls:
  /* Build our translation-unit decl.  */
  current_translation_unit
    = build_translation_unit_decl (get_identifier (main_input_filename));
because main_input_filename is null in this case.  We can fix it like
below, where main_input_filename will be set to "".

Bootstrapped/regtested on x86_64-linux, ok for trunk?

2018-05-10  Marek Polacek  <polacek@redhat.com>

	PR fortran/85735
	* options.c (gfc_post_options): Set main_input_filename.

diff --git gcc/fortran/options.c gcc/fortran/options.c
index 1405f1cf008..3c17a583f62 100644
--- gcc/fortran/options.c
+++ gcc/fortran/options.c
@@ -313,6 +313,7 @@ gfc_post_options (const char **pfilename)
       if (gfc_current_form == FORM_UNKNOWN)
 	{
 	  gfc_current_form = FORM_FREE;
+	  main_input_filename = filename;
 	  gfc_warning_now (0, "Reading file %qs as free form", 
 			   (filename[0] == '\0') ? "<stdin>" : filename);
 	}

	Marek