save_textarea_file: don't return a dangling pointer if open fails

If fdopen failed, save_textarea_file would free the filename, but it still returned the pointer to the filename.  With this commit, save_textarea_file will return NULL if fdopen fails.

src/viewer/text/textarea.c

@@ -305,7 +305,7 @@ save_textarea_file(unsigned char *value)
 		fwrite(value, strlen(value), 1, file);
 		fclose(file);
 	} else {
-		mem_free(filename);
+		mem_free_set(&filename, NULL);
 	}
 
 	return filename;