speller: don't crash when the spell-checked tempfile cannot be opened

This addresses https://savannah.gnu.org/bugs/?56361.

src/files.c

@@ -498,7 +498,7 @@ bool open_buffer(const char *filename, bool new_buffer)
 #ifdef ENABLE_SPELLER
 /* Open the specified file, and if that succeeds, remove the text of the marked
  * region or of the entire buffer and read the file contents into its place. */
-void replace_buffer(const char *filename, undo_type action, bool marked)
+bool replace_buffer(const char *filename, undo_type action, bool marked)
 {
 	linestruct *was_cutbuffer = cutbuffer;
 	int descriptor;
@@ -507,7 +507,7 @@ void replace_buffer(const char *filename, undo_type action, bool marked)
 	descriptor = open_file(filename, FALSE, &f);
 
 	if (descriptor < 0)
-		return;
+		return FALSE;
 
 #ifndef NANO_TINY
 	add_undo(COUPLE_BEGIN);
@@ -539,6 +539,7 @@ void replace_buffer(const char *filename, undo_type action, bool marked)
 	add_undo(COUPLE_END);
 	openfile->undotop->strdata = mallocstrcpy(NULL, _("spelling correction"));
 #endif
+	return TRUE;
 }
 #endif /* ENABLE_SPELLER */
 
@@ -959,6 +960,7 @@ int open_file(const char *filename, bool newfie, FILE **f)
 		if (*f == NULL) {
 			statusline(ALERT, _("Error reading %s: %s"), filename, strerror(errno));
 			close(fd);
+			fd = -1;
 		} else if (!inhelp)
 			statusbar(_("Reading..."));
 	}

src/proto.h

@@ -267,7 +267,7 @@ void make_new_buffer(void);
 void set_modified(void);
 bool open_buffer(const char *filename, bool new_buffer);
 #ifdef ENABLE_SPELLER
-void replace_buffer(const char *filename, undo_type action, bool marked);
+bool replace_buffer(const char *filename, undo_type action, bool marked);
 #endif
 void prepare_for_display(void);
 #ifdef ENABLE_MULTIBUFFER

src/text.c

@@ -2531,6 +2531,7 @@ const char *do_alt_speller(char *tempfile_name)
 	size_t pww_save = openfile->placewewant;
 	ssize_t lineno_save = openfile->current->lineno;
 	bool was_at_eol = (openfile->current->data[openfile->current_x] == '\0');
+	bool replaced = FALSE;
 	struct stat spellfileinfo;
 	time_t timestamp;
 	pid_t pid_spell;
@@ -2586,7 +2587,7 @@ const char *do_alt_speller(char *tempfile_name)
 									openfile->mark_x < openfile->current_x));
 			ssize_t was_mark_lineno = openfile->mark->lineno;
 
-			replace_buffer(tempfile_name, CUT, TRUE);
+			replaced = replace_buffer(tempfile_name, CUT, TRUE);
 
 			/* Adjust the end point of the marked region for any change in
 			 * length of the region's last line. */
@@ -2599,14 +2600,15 @@ const char *do_alt_speller(char *tempfile_name)
 			openfile->mark = fsfromline(was_mark_lineno);
 		} else
 #endif
-			replace_buffer(tempfile_name, CUT_TO_EOF, FALSE);
+			replaced = replace_buffer(tempfile_name, CUT_TO_EOF, FALSE);
 
 		/* Go back to the old position. */
 		goto_line_posx(lineno_save, current_x_save);
 		if (was_at_eol || openfile->current_x > strlen(openfile->current->data))
 			openfile->current_x = strlen(openfile->current->data);
 #ifndef NANO_TINY
-		update_undo(COUPLE_END);
+		if (replaced)
+			update_undo(COUPLE_END);
 #endif
 		openfile->placewewant = pww_save;
 		adjust_viewport(STATIONARY);